For the record, the current iPod nano is rated at 24 hours of continuous music playback.
To calculate how this battery size might translate into cellular use, I looked at the current iPhone 4 battery which is rated at 40 hours continuous music playback and 7 hours talk time. Doing the math, 24/40*7 gives you 4.2 hours of talk-time for a hypothetical iPhone nano of that form factor.
Long story short, I don't think battery life is the problem. Maybe it's too hard to fit everything in, but I wouldn't post this unless I thought it was feasible.