I think it’s Helvetica or Helvetica Neue, but I don’t know for sure. Here’s what I’ve pieced together:
Verdana is a Microsoft font. When Apple stopped bundling Internet Explorer, they made Verdana part of the OS install instead of the IE install so that web pages would still look nice and so forth. Ditto with Arial.
So you can assume that 10.4 has Verdana, but if you install 10.3 without installing Internet Explorer, you don’t get Verdana.
I tried rebuilding my app using Helvetica Neue as the default sans serif font; it was very close, but some of the text was too large.
For now, we’re putting a note in the readme telling people that you have to have Internet Explorer installed if you’re using 10.3.9.