It's not going to hurt if you have many many relevant ones. If you are just putting huge lists of them together that have nothing to do with the actual post/site then you may have problems in the future, not sure. Personally, I just list about 10 relevant ones and leave it at that.
The meta keywords are important for targeting adsense ads and not for search engine traffic. A friend did a test to prove this, check out www.pinkmingos.com/07172009 and view the page source then look at the types of ads that are on the page. The placement of meta tags are read by the Google spider and pretty much only feedback to the adsense placement.