I have no idea why the Info.plist entries are being ignored in Snow Leopard for calibre.app. Literally the only way I could get calibre.app to open up as an Intel (and not use Rosetta) on my Intel Mac was to do the following:
lipo -remove ppc calibre -output calibre.new
mv calibre.new calibre
Now, the program launches in half the time, the interface actually works properly, the calibre-parallel thread is used to do the work (unlike before). It still hits 100% of its cpu but on a multiprocessor system, it is just the one thread/cpu and the system stays quite responsive.
I am not sure why but from now on, I am going to strip out the ppc code from calibre so that it actually runs properly on my machine.
Just in case others run into the same thing.
Hope this helps,