I'd hang tight with Stardock they'll have complete Vista compatibility for OD that's for sure.
I think I got a reasonable amount of value out of OD... but IconDeveloper was probably 50% of it... I use a mixed bag of tools already, and if my regular raster image editor properly supported .ico, I'd just use that... it's just that I have an app going gold in the next month and can't be wrestling with an icon editor that doesn't properly support Vista icons... so unless a Icon Developer fix for 256 x 256 icons is due within days, maybe weeks, I have to buy a license for another Icon editor (my "Real World Icon Editor" trial version still functions, but I can't in good conscience use it for production-quality icons without paying for the license.
...and given that StarDock is big into Vista skinning etc. I can't imagine how Icon Developer hasn't been upgraded already... unless their skinners use other tools to do their work (gasp!)
I'm also willing to concede that I might be doing something wrong, or have an incompatibility: no one has piped in to say "I have version x and I don't have that problem: I design, open, and save 256 square icons no problem!"
So, I assume it is a hard limitation (bug?) in Icon Developer, and no fix is forthcoming, and I'm not the only one in this situation.
So I guess I've answered my own question. Thanks, me!