I have XP MCE 2005 V 5.1.2715.3011 and I think I'm stuck.  I recently "donated" My 2 ATI (eHome Wonder and HD Wonder) tuner cards since they were running very hot and I had ordered an HD Home Run.

 It seems I needed to keep the useless analog tuner for setup? 

 Settings/TV/Set Up TV Signal/Region=US  yields "UNSUPPORTED REGION"

Is this really because I have no analog tuner?  Is there a work-around?

Thanks.

What type of signal are you connected to? 

HD homerun supports ATSC and clearQAM(digital unencrypted cable)...  I don't think Media Center supported QAM until Vista + TV pack. 

 I'm connected to antennas.  I can watch TV via the Silicon Dust HD Home Run software, but I think I'm screwed with Media Center XP.  :-(

You're correct - MCE2k5 requires that a supported (read:  Media Center-compatible) analog tuner be present to run TV Setup.  There is no workaround - sorry.
While you can later pull the tuner out of the system (after you've run TV Setup), I wouldn't sell it (the analog tuner) - should you ever need to re-run setup, you'd need to reinstall the analog tuner.

HTH,