Cloaking is strictly prohibited by search engines. They likely won't find cloaking on their own, but if someone reports you, you would most likely be penalized. Usually if you have to ask how much something would hurt seo, it should be avoided.
Yup defo big no no, I know someone who accidentally left some hidden links in his code, total accident and his site has been hit hard by the search engines ! Woc
This cloaked site www.peoplemagazinesubscription.com is on 3rd position at Google.com with the search term "people magazine subscription". Also the site is reported few times to Google.com for cloaking but it is still online and on top position from 5 months.
In what way is that site cloaking? There are different kinds of cloaking. When the cloaking appears to have been done for the visitors alone, the search engines would likely let it slide. Google has been quite slow with dealing with spam reports. They might've had to lay off some employees in that department when the recession hit. So perhaps those spam reports are still in the queue. You can get away with violating Google's rules for a while, but eventually it catches up with you. Usually it is not worth the risk.