I went ahead and put my Document!X on Vista a couple of weeks ago, and have had no problems till today. Adding a hotspot to an image (.png or .gif) in a content file crashes the Doc!X IDE. I tried the same action in the same project on XP and it works fine: apparently this is a Vista bug.
I'll tell Innovasys support (and have already uploaded any number of crashdumps!), but in the meantime, if anyone's got a workaround to this, please let me know!