Sure, you should have META keywords and they need to be structured properly, but you should know where META keywords rank in the scheme of things. Alta Vista seem to have gone a bit haywire and started to decrease relevancy for repetition somehow. My recommendation is 2-3 repetitions within the META keywords tag. If every one of your keywords was a single word then you wouldn't need to. However, you're almost certain to have keyword phrases on your list which you'll be wanting to enclose with commas to show the phrase as a single entity.
Semicolons are no better than commas.
That's an accepted fact. In the most extreme cases, a whole domain gets blacklisted by the search engine staff. Weeding takes place on a regular basis. The staff do actually check their databases and kick out semi-spammed documents, so keep popping back to check your listings. Infoseek is a case in point at the moment, the odd prize rose bush has been disappearing lately in their weeding frenzy.