Have had the same experience with both vegas pro8 & pro9.
Pinnacle Avid 7.2 also behaves in the same way. SD works perfectlty but HDV cannot be written back to tape.
My Vegas progammes are 30 day trials as I am looking for an alternative to Avid 7.2 which will not be supported in W7.
My pc is dual booted (separate drives) with XP Pro & Windows 7 (RC7100).
Both Avid 7.2 and Vegas work OK with XP so the likely problem is the MS driver provided in Win7.
Hoping this is corrected in final release.