I did that and it seems that it's only downloading such amount of data when it's on 3G. The wifi doesn't seem to have this problem.

I didn't watch anything or had any video calls. The only fact that I am connected to the 3G and the counter starts running (using the Dataplan monitor).
This is a new problem that came after the Maemo update, it looks like...

Here how it shows on the "recent use" T-Mobile page:
29/05/2010 20:16:12 1000 TMO Network Coupon 2101 QOS1000 181 Kb £0.00
29/05/2010 20:15:24 1000 TMO Network Coupon 2101 QOS1000 1601 Kb £0.00
29/05/2010 20:12:18 1000 TMO Network Coupon 2101 QOS1000 1602 Kb £0.00
29/05/2010 20:08:48 1000 TMO Network Coupon 2101 QOS1000 1601 Kb £0.00
29/05/2010 20:05:26 1000 TMO Network Coupon 2101 QOS1000 1601 Kb £0.00
29/05/2010 20:02:26 1000 TMO Network Coupon 2101 QOS1000 1601 Kb £0.00
29/05/2010 19:59:19 1000 TMO Network Coupon 2101 QOS1000 1602 Kb £0.00
29/05/2010 19:56:04 1000 TMO Network Coupon 2101 QOS1000 1601 Kb £0.00