1. Applications Development Framework (ADF): it seems like everything…and I do mean EVERYTHING…in Fusion Apps was built in ADF. Even advanced UI Personalization in Fusion Apps requires some knowledge of ADF.
2. BPEL Process Manager (BPM): you'll be using this to build customer-specific business processes, leveraging the abstractions (especially the entity objects) and the services built into Fusion Apps. To be honest, I'm really a fortunate neophyte here. I didn't know jack about BPM a few months ago, but my colleagues at Innowave are bringing me up to speed quickly (they really are among the best at this stuff - I'm a lucky boy playing sponge with these guys).
3. WebCenter: all the collaborative workspaces in Fusion Apps are built and accessed via WebCenter. Portal, Composite Apps, Collaboration and Social functionality, Content Management…yeah, you better know this one.
So if you've been keeping up to speed with Fusion Middleware tools as they've become available in the Applications Unlimited products, you're in pretty well prepared for the apps technology in Fusion Applications. However, if you're still working with older tools (yeah, I'm talking to all apps developers still writing apps customizations in PL/SQL), you need to get moving on improving your skillset.
In fact, now that I think about it, you need to get going whether or not you're interested in Fusion Apps - these are the tools of the future for all the Oracle Applications. Feel the fuse burning?