If you want AT&T's coverage (for voice) on prepaid, the best option is
to use 7-11 SpeakOut which is 15¢/minute, and uses the AT&T GSM network.
This is a lot more expensive than T-Mobile prepaid, but the coverage is
much better (but not as good as PagePlus CDMA). Note that you have to
buy it from a 7-11 in the U.S., the Canadian SpeakOut service doesn't
work in the U.S.. Also, you must buy a phone, they don't sell just SIM
cards, but you can use that SIM card from the phone in an unlocked
phone. They charge $1.25/month fee, and you have to add a minimum of $25
every 365 days. I keep a SpeakOut phone just for coverage checking so I
can keep my web sites on wireless accurate.

I know you want to stay on GSM, but you still might look into PagePlus
as it really suits your usage model.

With PagePlus prepaid (CDMA on Verizon) there is currently free (albeit
slow) QNC (14.4K) data but no guarantee on how long this free data will
last. See "http://www.howardforums.com/showthread.php?t=1271960".
However apparently PagePlus is planning to bring non-accidental, paid
data services to prepaid later this year (I guess if Verizon allows them
to sell EVDO data services). Obviously you don't want to do much web
browsing at 14.4K, but for occasionally checking e-mail it'd be alright.
PagePlus doesn't roam in Canada (at least not in Alberta).

The huge advantage of PagePlus is the rural coverage, both on CDMA and
AMPS. AMPS is still going to be around in rural areas long after the
carriers are permitted to turn it off next month. I've had PagePlus
service, either CDMA or AMPS in many rural areas where there was no GSM
coverage at all, and even just comparing CDMA versus GSM, barely a month
goes by where I'm not out in a national or state park somewhere where
there's no AT&T GSM coverage, but there is CDMA coverage. Last weekend I
was up in Lassen National Park, where there was CDMA coverage while XC
skiing, but no GSM coverage. I was surprised that I wasn't on AMPS as is
usually the case when in a U.S. national park. PagePlus is very
inexpensive, as low as 5.3¢/minute, though if you go off of Verizon
you're roaming at 59¢/minute (fairly rare, and you're warned by having
to double dial). You have to add a minimum of $10 every 120 days.

See: "http://www.prepaiduswireless.com/" for details on both 7-11
SpeakOut, and PagePlus.

We use T-Mobile prepaid phones and a few years ago also picked up a used
Sidekick II on Ebay and use it only when we go on vacation buying a $10
refill card as needed.  Trick is is to refill every 90 days otherwise you
loose your number.  It has a browser and can get email from up to 3
accounts.  The browser is kinda clumsy but we manage to check our portfolio
with no problem.  It cost $1 / day but better than lugging the laptop
around.

Recently we picked up some new quad band phones from their web site which
have world band and the newer band that covers more areas in the US The
phone is the Motorola V195 s and can be seen on their website
www.tmobile.com under plans, prepaid, prepaid phones.  It sells for $40 on a
web only deal but you get a free $25 refill card which we had to call for as
it never came with the phones.