The first E89 Z4 will be delivered in the May/June 2009 timeframe. I don't think that BMW is planning any upgrade/fine-tuning for its MY2010 model that will come out less than six months after the initial release of the new model. It would seem to me like bad publicity. So, no I don't suggest you wait for the 2010 model.
However, if you are looking to save a year of depreciation then in that case the it seems like it's a good idea to wait for the 2010 model which will most probably be identical to the 2009 apart from possibly new color combos. It depends what you're after.
Cheers.