If i can permit myself to speculate...
Here's a quote from nVidia developer website:
A driver that enables NV30 emulation will be released shortly.
What if that was NV30 AA method, and that by error, in 31.00, it was done in software since there's no capability on NV25s to do that in hardware?
If that's the case... Screenshots maybe? Might be interesting to see actual AA performance, just in case i'm right ( hey, i could be wrong, i'm just speculating )