Isn't that just like switching on 2G ?.
'GSM Only' limits you to 2G, and apparently 'GSM auto (PRL)' works with 3G/HSDPA. But importantly I haven't had mobile internet activated since switching to GSM Only and I've still seen quite a massive difference in battery life.
EDIT: Yeah 'GSM auto (PRL)' gives me HSDPA speeds, and apparently still saves battery power compared to the default setting in 2.1 ROMs.
