Got connected with Optimus 3G prepaid internet through a Nokia N95 8Gb using a data only simcard. Took a bit to get the sim only in Viana do Costelo - all but 1 shop tried to sell the 3G USB stick.
There's a few things to know about the Optimus deal but so far it does work great. Coverage seems to be ok - 3,5G where ever i tried. According to locals Optimus seems to have the better 3G network.
Read all about it :
Fast 3G Internet in Portugal