I am coming in to Hahn airport and need to get to Frankfurt Airport at 11 PM (2300). What's the least painful way, i.e., is there a bus that late? And if not, is there a decent(?) hotel in the area I can stay overnight and get to the Frankfurt Airport early in the AM?
Buses run from Hahn and generally meet incoming flights. It'll take a while but you'll get there...
For future reference, the place to begin looking for answers to airport-related questions is the airport's own Web site.
It's a 90-minute bus ride from the Frankfurt-Hahn airport to the Frankfurt airport and another 15 minutes from there to the main Frankfurt train station. Here is the bus schedule.
The Frankfurt-Hahn airport site has a list of area hotels. See here.
The bus schedule from Frankfurt Hahn airport is here. It takes 1h45m.