Not at all, Creo 3.0 will open both parts and assemblies (without features history, of course) and allow to work with it. So far it's being show in assembly context to show how users can now insert native CATIA, NX and SW files directly to assembly without converting/importing them to Creo format, but nothing stops you from opening a single part in Creo 3.0 and working with it (mainly using Flexible Modeling Extension, if you want change part's geometry). And it's part of standard Creo 3.0 functionality, without additional fees. You have to pay for additional extension if you want to save files to CATIA, NX or SW format, though.
Check this screenshot from one of video presentations:
