Well not everybody uses NVIDIA cards. Besides that I to this day didn't read any solid facts on what NVIDIA exactly wants to do with PhysX - maybe they'll just put extended support for it in their Quadros or stuff. I wouldn't base the decision here on some "maybes" and "probables"...

It might also be a matter of price - I'm not sure about it but afaik using PhysX does actually cost you a lot. The SDK may be "freely available" but I don't think it will be without charge if you use it in a commercial product (Ichiro's quote of Sony having "licensed" the SDK sounds pretty much like that)...

With Bullet there is a free, professional and powerful engine and if you like having NVIDIA work on a physics engine they to some extent support bullet too...

Concerning the PhysX driver issue:
Apart from the fact that this extra need to install a driver is annoying there imo also is the problem of this being an additional source for errors of all kinds. I mean if the physics engine is housed in Acknex itself you're much safer when looking at the bug side. You won't get strange behaviours on some PCs with new updated drivers or have to do a more extensive bughunt - with the physics engine being in Acknex you can make sure everyone uses the very same engine and are safe from possible problems an external driver provides...

I also think kind of a new philosophy shines through here now. I mean to give an example look at that whole shader discussion. Even at times of a GeForce 5 shaders didn't have a high priority because we got told that the "average user" wouldn't have such a card yet and Acknex isn't meant to mess with the AAA titles and their features. So why suddenly things like that are considered and we even discuss things that people might eventually have with GPU generations still to come? I mean we really were often told that this still is an Indie engine so with defining your product this way there should be no big discussion about PhysX being an option...

With that said I think Bullet provides really lots of things: It's multi-threaded, doesn't really lack anything an Indie misses, is supported by many companies and used in big commercial titles too and it also might be familiar to some degree for those who use Blender (which especially in an Indie community with many non-commercial developers is widely used)...

So maybe we should check the point of the costs first because I'd rather like to see some money spent on different areas of the engine instead of physics...

Enjoy your meal
Toast