it's a reason to be banned fro google search . you can read the official answer on http://www.google.com/support/webmasters/bin/answer.py?answer=35769
Quoted from Source: "Cloaking, technique of serving a different page to a search engine spider than what a human visitor sees. This technique is abused by spammers for keyword stuffing. Cloaking is a violation of the Terms Of Service of most search engines and could be grounds for banning."
you will ban if they caught you. so the tip is if you want to cloak a website don't tell to anybody that your cloaking.
Cloaking is a blackhat technique in which the site that was presented in the search engine was different from the site that was presented for the user.
Honestly I think whether cloaking is bad for you depends on what you do with it. A lot of sites are starting to use cloaking to display content to people of certain countries and different content to others. Someone said something yesterday that really rang true with me: "People talk about cloaking a 'unethical' -- as if Google is the final measure of right and wrong." I'm paraphrasing of course, but it makes me wonder. I wouldn't use cloaking on a site that you're working hard at (use it on a new domain with private registration), and of course don't use adsense or google analytics on a cloaked site. Using adsense on a cloaked site is the easiest way to get found out using cloaking, so avoid that at all costs.