This is probably well known, but I'm posting a personal experience to flag this for people who haven't come across this problem before.

I spotted a quite well searched keyword that had little competition and was highly relevant to my site - I just hadn't thought of it before. I decided to include it in my main page's copy and of course the keywords (metatags). The mistake I made was to put it in the keywords a three weeks before I updated the copy.

It first my site appeared at around 500 for a while, then one day it disappeared (went to a position >1000) for that keyword, and updating the copy to include it has made no difference in several weeks.

So don't be lazy, update your copy at the same time as the keywords.

Now, on Google, I've probably got a three to six month wait. On Yahoo and MSN, the position is climbing nicely.

Fortunately all the other keywords are unaffected, so it's not a site penalty.

Argey