I have seen a site ranked very well on google and that site has hidden text in the bottom. That text is the keywords , around 12+ keywords and the text is in the same color as the background and its invisible unless you see it with x ray vision. i mean try selecting the content then you can see in blue. I guess this is spam. Initially i was sure but i discussed the same in google product forums and they(only the top contributors) didn't accept that its spam but general guys who are on level 3 or so totally agreed that its spam.
No, it's not spam but it's not white SEO either. The thing is, why you should hide them, it's enough to do a "tag cloud" Hope you know what I mean Cheers
The term "SPAM" is really broad a generic to the SEO world, but to answer your question, YES! Just don't do it, even if you see others doing it...Sooner or later, they will be shot down! Holla!
Such keyword practices we're one of the many things BMW got slapped for by Google back in 2006 But as for it being Spam the first reply above pretty much sums that issue up...
Google is software, not manually review sites, so maybe the spiders haven't found it yet! When they do though hidden text isn't a permanent ban or penalty, you should receive a message in your google webmaster tools to remove them and alleviate the ban!
Google just havent find it yet, but its a blackhat technique and sooner or later they might get a bad result from it...
If you are trying to make a webpage useful for your visitors, then why to hide your keywords from them?????