Excellent idea!
For one thing, it's relatively painless to do from a technical perspective--all that's required is re-ripping the same songs; no new artwork/ID3 tags/etc. needed. Sure, they'll need 50% more bandwidth/server capacity, but I gotta figure that they're already beefing this up 5-10-fold for the Windows version already anyway (not to mention doubling the number of tracks available).
In addition, depending on how they do it, they might even nab some extra cash from everyone who's already bought tracks--the more serious music lovers would probably be willing to pay, say, another $0.25 to "upgrade" each of their tracks from 128 to 192. I could see something like $0.99 for 192, $0.79 for 128, and $0.25 to upgrade existing 128 tracks to 192...
...or would this make the whole thing too confusing for new users? Maybe just change every track to 192 and keep it at $0.99 would be better.
The *only* downside I can think of is that it would mess up their iPod advertising a little bit--the "10,000 songs in your pocket" slogan is based on 128 tracks; at 192 it'd be "6,500 songs in your pocket" which is still great but not *as* great...