The 'stimulus package' has been signed into law, and NIH has decided how to dole out the money. Information can be found here: http://www.nih.gov/about/director/02252009statement_arra.htm
In short, those research grants already in line have a much better chance at actually being funded than they would have just a few weeks ago. Existing grants seem to be in even better shape -- they will have the opportunity to be expanded in scope. Both of these mechanisms allow NIH to get the money out fast.
There is also a mechanism being put into place called "Challenge Grants". Currently the main NIH page for info on these grants states "Information Coming Soon" (http://www.nih.gov/about/director/02252009statement_arra.htm). I guess we'll have to wait and see what those are about.
From my perspective, this stimulus package bodes very well for investigators already funded or on the verge of being funded. Anyone not already in the mix isn't likely to benefit.