My dashboard has a whole lot of play up and down, but the nuts holding it on seem like they don't tighten any more. What's going on? I can see the ignition bit rotating as I pull the dash 

There are some rubber grommet/bushing type things that belong in there. The nuts bottom out on a shoulder and should bottom against the grommet. Plus there's a washer that belongs on there to go against the grommet.

It should have a little bit of flex but not be loose. I'd check and make sure the grommets are there, not broken/cracked/ chunk missing.

I compared it to my other GS500 and everything was there but the flex was far too much. It seems like the rubber grommets are pushing against the nuts pretty hard, which might have prompted them to loosen a bit over time, and it just takes a bit more effort to tighten them back because you've got to press it against those rubber grommets that are resisting the nut. I tightened it bugger-tight and it seems to have fixed it.

The bugger was loose again this morning, might try crushing the rubber with some vice grips or something when I tighten it back up this eve.

The bolt has a square bottom profile. That sits in the square hole in the dash's top plate. If that's not tightly in there, it could refuse to tighten.
Cool.
Buddha.