This issue was brought up fairly often when the 99 Mustangs first came out. My '00 GT has a little bit of road noise from the rear, as does every 99-01 Mustang I have ridden in (mostly V6 rentals). It seems worse in the V6 models to me, probably because the exhaust is quieter than in the GTs. My girlfriends 98 V6 also has a bit of road noise from the right rear, so I don't think it's just the '99+ models that have it.
The bottom line is that it's "normal" for late model Mustangs. I'd guess that Ford skimped on the sound deadener to save money and/or weight. Of course I have heard stories of the outer wheel bearings in the 8.8" rear end wearing prematurely in these cars too, so if the noise gets worse it's worth looking into.
'00 Mustang GT
'72 Camaro 383
'87 Porsche 928 S4
'93 F-150 4x4 5.0L