The pages that I have been working on for the past few months just got deindexed from Yahoo. Its weird as all the other lame pages are still up, but the ones I put effort and work into are gone.
I feel ya jdeeze, I'm pulling my hair out with Yahoo as well. Was ranked in the top 5 for a very popular keyword - and now its down to about 50. Whatever yahoo has done...I wish they would reverse it
I think the main problem is there isn't much SEO information on Yahoo and MSN when compared to Google. One can guess that the basics are the same, but who knows.
Many people reported same problem as you, even for google, same problem happened again and again. Wait a few days, if nothing is better, you shall contact yahoo for the reasons, or try to submit to their directory.
It's probably because of the changes you made to the page, I've noticed that with yahoo. Like others said, you should wait some days and see what happens.
I find Yahoo is a lot more volatile than Google, especially when your site hasn't been sitting on its rankings for a while. a couple changes on the page can throw off the whole dynamics for Yahoo