We are setting up a new SharePoint 2010 QA server and would like to use the same SQL server as our SharePoint 2007 QA server. Are there any issues or gotchas with this strategy?
That should work just fine. You don't need separate SQL instances either, just make sure you don't do the 'all in one, let the SharePoint installer sort everything out' install.
This is assuming that your SP2010 and SP2007 servers are 2 separate (virtual) servers talking to the same SQL database server.
There shouldn't be a problem as long as you use separate SQL instances for 2007 and 2010 on the same server. They will operate independent of each other so no problems should arise.