There's a HUGE thread on this already. The Solstice transmission IS made by Aisin, and IS similar to the R154, but it has a different spline input, different tail shaft, different shift linkage, different gear ratios, different synchros.
Aside from bellhousing bolt pattern, and general size, they're completely different. There was a guy that modded one in to a Supra though.
As for teh Solstice having a 6 speed, people may be thinking of the GXP model.
