Our main site right now is on SharePoint 2007 (WSS 3.0), no MOSS or other fun stuff. At some point it will move to SharePoint 2010. There has been a suggestion that we do that in the next couple of months. Our concern is that we suspect that a number of our custom developments might not survive the migration, there are somethings we can do with ease on SharePoint 2007 that simply fail if we try to do them on 2010, for example graphical alerters in place of numeric fields. Given the number of bespoke developments we have migrating could be a very painful experience and we don't have the luxury of downtime, the sites have to be available.
I did find a forum thread on LinkedIn that discusses the move. It seems that there's a tool (STSADM) that will pick up most of the problems you're likely to face.
The thing is, most of the sites in our site collection (including the ones with the most bespoke development) will be getting archived in March/April/May 2011. If we wait until after this has been done there will be a lot less to go wrong and to fix. I have recommended that we wait until then. Yes, 2007 is very out of date but we do not currently have any problems that would be fixed by going to 2010 or a business need for any features added in 2010. We do have a 2010 site so if a new site came along we could build that on 2010 and if a site developed a need to move to 2010 we could transfer just that site over.