I know this is getting toward heresay, but I also had this problem with my 80gb ipod. I just persisted with the hard reset sequence and it eventually worked. A couple of times I really thought I was in deep fecal matter.. but generally I could get out of it by enough persistance, the odd bit of extended charging and hard resetting. I think it all came down to Rockbox being the culprit. It would screw the battery measurement, run the battery until it was completely flat, and then crap out. Normally, under the Apple/iTunes OS, it'll restart because there is enough juice remaining to boot.. but after rockbox has sucked the battery dry due to dodgy power measurement, it wont start up. I don't know what it's like now, however.. there's been lots of work done on power libraries since I uninstalled.
I didn't uninstall Rockbox due to this matter, but due to the consistent never-ending Hard Disk corruption, cross linking and other false file dumps the Rockbox os would generate on my 80gb ipod. I ended up losing my entire ipod_control dir due to it becoming crosslinked, and (probably due to my own damn fault) chkdsk /f then wiped those crosslinked files upon repair. Generally I'd need to chkdsk every 2 weeks to clear off directory after directory of garbled nonsense, and massive 4gb "files".. maybe the FAT corruption was something to do with the way the new 80gb HDD was being read or written, I don't know. However, I'm not going back to Rockbox until its sorted because I don't want this to keep happening. Its sad, because Rockbox is such a great project and a better tool than itunes.