(Of UK interest only)
(Shoot me down in flames if it's obvious and I'm the last person to realise.)
I just discovered the experimental Highways Agency http://www.trafficradio.org.uk now seems to be 'live'. The streams won't play on anything I already had so I forked-out �15 for LCG Jukebox, which plays them quite happily, albeit with the occasional ear-piercing squeak. The low quality streams are perfectly audible and seem to work over the crappy GPRS I get in my area.
There seems to be just about enough RAM to have this running alongside Maps, so I suppose you could have it running in the background paused, and switch to it from Maps every so often to see what's going on.
Of course, Maps should have live traffic information but doesn't seem to. So until it does, this could be an alternative.