I'd be very surprised if Motion doesn't at least allow for QT output, meaning you can use it for any version of FCP in that sense (which is how LiveType works--worked?). If you want complete integration, you may need FCP HD. I have no problem with that. Just speculation on my part tho.
On paper, Motion looks not like a replacement for AE, but a damn fine alternative for a given project. For example, LiveType destroys AE for general titling. Even with AE's new excellent type tools, it can't touch LiveType for your basic cool looking title done quickly. It looks like Motion will do the same for your typical broadcast intro/outro, title fly-ins, logo work and similar, but AE will hold its own for a few more years with motion tracking, Z-depth, parent-child relationships, scripting, multi-format export, etc.
I suspect Motion may allow me to do nine out of ten projects using it, but that 10th project will absolutely requires AE's power. Either way, I win. Unless Adobe pulls the plug on AE some day. Ugh. While I'm thinking about it, the only way Adobe will pull the plug is if they don't sell enough Mac copies to profit from it. It's that simple. They're not going to stop development out of spite. It'll be up to us developers to determine the fate of AE. I predict it stays on the Mac for a good long while.