If you are referring to E90 3 series, then you are looking at about October 2005 release for Sedans. The Coupes will follow in spring (March?) of 2006.
At least in my region, BMW's do not usually sit on the lot for a long time. Also, depending on E90 'styling', people may prefer to buy current E46 instead. In that case, you may actually have to pay little more to secure the last few.
It is a gamble. For me, I really like current E46 styling. No complete E90 pictures have surfaced but little bits and pieces shown on internet so far, only thing that interests me is potential larger power plant (which is not likely to show up until several years into the model life).
In any case, whenever you choose to buy, they will always have something new the year after. It is a neverending chase. If you wait for the best, you will be waiting forever.