Just got back from a week's vacation. Drove hwy 5 to 97 at Weed, CA. Then 97 to Bend, OR. Unlike last year, this time I had coverage all the way on the ATT GSM phone. While in Bend, the provider read as "Voicestream" (aka T-Mobile). We stayed with friends in Bend who then took us up to Sisters (so my wife could "shop til she dropped"). And then we went beyond that into the thick of the forest to some falls. Even there I had coverage and so did my wife's TDMA phone.
Yeah, Hwy 97 should have GSM coverage. A problem area for GSM are Hwys 58 and 138 that run E-W and connect I-5 to Hwy 97. TDMA coverage is very good in most of Oregon (haven't checked in southern Harney and Malheur counties though).