The Terminator 3 case says it has Dolby Digital Plus, but I didn't see that as an option on-screen (just Dolby Digital). Did I miss it?

Yes, the HD DVD is Dolby Digital Plus. However, its Warner's business as usual 640k DD+ track.

The Blu-ray sports the same soundtrack but its not labeled as DD+.

Dolby Digital Plus is the default standard for HD DVD. However, the format can be encoded at either 640 kb/s (which Warner uses) or 1.5 Mb/s (which everyone else uses). The 640 kb/s variety is only a small improvement over DVD.

After hearing other DD+ tracks and then popping in T3, I was very unimpressed by the T3 disc. I had to turn volume way up to get the same oomph I was used to from other DD+ discs. Now I know why. Thanks.

If DD+ is 640 kb/s and DD 5.1 is 640 kb/s is there any difference at all?

well, very few, if any DVDs have a 640 kb/s DD 5.1 track, as far as i know. but to answer your question directly, no, there should be no audible difference. it's just a different way of compressing the exact same information (theoretically).