Testresults IM:
1. Skype idle 8h on 2G: Battery 97% down to 82% = 15% in 8h = 1,875% per hour.
2. Skype idle 8h on 3G: Battery 95% down to 79% = 15% in 8h = 1,875% per hour.
3. MSN Idle 9h on 2G: Battery 97% down to 74% = 23% in 8h = 2,555% per hour.
4. MSN idle 8h on 3G: Battery 96% down to 55% = 41% in 8h = 5,125% per hour.

Also Idle with nothing online:
8h on 2G: Battery 96% down to 89% = 7% in 8h = 0,875% per hour.

MSN Messenger eats your battery! But its reasonable on 2G.