Okay, it seems that the problem lies with my templates; if I recreate them from scratch, I don't have the TNR problem. It's only when I try to resave/overwrite and existing template with an updated version that I get the TNR problem.
Yes, I've done all of that. The problem is the two hard returns Outlook inserts before a signature—they default to Times New Roman (and I'm not sure where or if I can change that) and that's what changes my font.
The workaround I've come up with is to insert an extra hard return after my name...
Diane, I also have this issue. I moved from Outlook 2003 to 2010 and copied over my templates folder. When I open the templates in Outlook 2010, the last few lines of the template (my signature, but not the signature Outlook appends), for example: