I would be trolling market.android.com right now to get a feel for apps. That way day one you will be good to go. Also, if you don't have a gmail account, get one ready because you will definitely need one. Take a look at videos with the new sense 2.0 so you can get a feel for the OS UI and curb the learning cure. Maybe look into accessories such as an SD card. Cases are coming out slowly, and while most people believe the Desire HD cases will fight the Inspire, I am still skeptical because of the curvature on the Inspire.
I would wait on ROMs. They are device specific and while most devs will probably mod their Desire ROMs, it may take some time to get it all working right.
Oh, also syn your contact with your gmail account so you can load them instantly to your phone without having to deal with the retail store doing it.
Other than that, I think just wait and get ready.
EDIT: Just say your Gtalk info in your sig so I guess you have gmail already lol