It is not going to be as big as Google but it will still have a fair number of users so advertising on Bing will still be somewhat effective.
Recently read an article at chitika.. might good for you. http://chitika.com/blog/2009/07/24/chitika-analysis-bing-traffic-vs-google-and-yahoo/ Microsoft is behind bing and it seems to first ever 'serious' try of M$ to make presence in search engine market.. so not counting bing might cost some bucks in near future.
Evidently it is hard to rank in Bing. I have really lost a lot of activity since the move from MSN, where I had many page 1 ranks for two-word keywords.
I am continuosly impressed by how quick and efficiently Bing retreives data. It seems to be wicked smart....