Classic quote of the day, first mentioned ca. 2004 on creativeCOW by esteemed colleague Rick Gerard: "Those who tell don't know, those who know don't tell". If Mercury ever comes to AE, you will clearly hear the marketing train making noise. Until then you are merely wasting your time with idle speculation. Adobe never gives away features of new versions until they are ready and since CS5 has only been released a few months ago, it is really pointless to ask what may happen next. if you allow me - it is as productive as asking your mum and dad as to when NASA will finally make this long promised manned missiojn to Mars... Could happen tomorrow, could be never...
Mercury is three things:
1) 64-bit application
2) multi-threaded application
3) CUDA-accelerated rendering of some items
After Effects CS5 has #1 and #2. Some plug-ins, like GenArts Sapphire, have #3. So, how about that as a "we have it now" answer?
Of course, I'm pretty sure that what you were really intending to ask is whether the After Effects application will ever use CUDA to accelerate some operations. Maybe. If it's important to you, make a feature request and tell us what you want and why and how you expect it to work.