According to bitcoin.it [1] iPhone 3g's cpu (ARM1176JZ(F)-S) can achieve 1.19 kH/s
When I plug that into Alloscomp's bitcoin calculator [2] it says that at todays exchange rate you could expect a return of about $0.02 per month per phone (assuming they're on 24/7)
If you had one billion devices you would make around 30 million per month. One billion devices is totally unfeasible though (Especially one billion on 24/7) and you could definitely make more money some other way with that many devices.