I got this mail from google. Everyone who using Adsense got same mail.

Hello,

We are writing to share some important information with you about
referrals to the AdSense product. As part of ongoing efforts to
optimize revenue opportunities for our publishers, we're
constantly experimenting with new revenue-enhancing features as
well as tweaking those products already available to our
publishers. This is the case for referral units directing visitors
to sign up for AdSense. After experimenting with this program
over the past year, we've concluded that there are other products
that are of higher value than this program to publishers in your
region. As a result, referral units for the AdSense program will
be retired in the coming weeks. Referrals to other products and
services remain unaffected.

If you're currently displaying referral units on your site
directing users to sign up for AdSense, read on below for details
about what to expect in the coming weeks.

In early January, the option to add referral units directing users
to the AdSense product will no longer appear in your account. You
will continue to accrue earnings for all existing referrals yet to
generate $100 until late January, at which point the program will
be fully retired. Existing referral units will continue to appear
on your pages.

By the end of January, you should remove all referral units
directing users to AdSense from your pages. Referral units
that you do not remove will continue to be displayed on your pages
as normal, but conversions will no longer be recorded. We
suggest you replace the AdSense referrals with referrals to
another product or service or an additional ad unit.

Thank you for referring users to AdSense in the past. We
apologize for any inconvenience this news may cause.

Sincerely,

The Google AdSense Team

Google Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043

I am not using this in my any site or blog this time. I used it once in my blog where I got just one sign up. Then I removed it. I don't think Google need this referral program because they already getting lot of Adsense Users.

Are you still thinking to cheat AdSense? Stop that.
It will never bring you anywhere. You can do this with smaller ad network but not with Google.
Here are some of the detection methods Google might use to catch cheaters.
At the very least, they have the resources to do so.

IP Address
If the AdSense click is originated from the same IP Address as the one used for accessing your AdSense account, your account is flagged.

Cookies
Most home users do not use static IP Address for Internet connection. In most cases just disconnect and reconnect will give you a new IP Address. But don?t forget, Google has set cookies on your computer.

Other Google Services
Thinking that you are safe just because you do not access your AdSense account? Think again. This time, consider these: GMail, Google Earth, Google Calendar, Google Search, Google Toolbar, Google Talk, Google Sitemap, Google Desktop, Blogger, and so on, and so on. With the wide range of services they provide, Google can trace the originator of most (or probably almost all) clicks.

Click Pattern 1
Oh, why this computer / IP address / person is so trigger-click-happy on this particular website but never click on the ads on other sites?

Click Pattern 2
And why is it that people accessing these sites direct (type-in URL or from bookmark) tend to be very active ad-clickers compared with those referred from search engine or other sites?

Click Pattern 3
And why the ad-clickers like to hit and run, compared with non ad-clickers that surf a few pages before leaving?
Click-Through-Rate (CTR)
Your CTR may range from 0.5% to 10%, but if it exceeds a certain point (probably around 10%), you are flagged.

Geo-Location
Used Urchin (Google Analytics) before? Then you should know that Google can trace traffics origin down to the small town. Different IP doesn?t mean much. Unless you site is really targetted to one small geo-point, a high number of clicks from nearby location will get you banned quickly.

Hardware address?
MAC address of the LAN card, modem, and router works almost like a fingerprint. I?m not sure if Google can track this, but probably they do. They have rocket scientist, remember?

Advertisers conversion rate
Ad click is one thing. But does it bring value to the advertisers? If none of the clicks on your site translate to conversion to the advertiser, you are in trouble. First the Smart-Pricing hits, then your AdSense account disabled.

Search Engine Ranking
Your website is not indexed on any search engine, not linked by any prominent website, but get consistently high traffic? That sounds like something is in play. Regardless of whether it is an adware-embedded software, spam, trojan clickbot, or intentionally installed click-exchange network, it doesn?t sound right.

Webpage design
How about the ?click here? or ?support us?? Google has the best search engine in the world. Is it really that hard to find those words?

Combo
Each of these detection methods might seem rather weak. But combine them together, and not many click-fraud can pass-through these filters. Even the smartest clickbot will have a hard time.
In short, it is impossible to cheat AdSense for a long term. Instead of spending time, money, and effort trying to outsmart Google, try some tips to improve your AdSense earning.
Note : I m not working for Google nor in anyway know anyone inside Google. Google might or might not use these methods to detect click-fraud. I think believe that they have much better detection mechanism.

Source of many available sources on net about this topic.