i don't believe the 5gb cap is confirmed as of yet, more of speculation. either way i have used pdanet extensively while traveling for businesss and never incurred a charge. i wouldn't be downloading videos or crap like that, but if you are just going on the internet to send email or surf you won't have any issues at all.
I've been tethering since I got my iPhone with PDANet and I have gone over 5 GB this month. When I signed up it stated unlimited data and I did not see one mention that tethering was not allowed in the TOS.