Find  a hand held like a ipaq or palmtop, which can accomodate a
mobile network card like Sierra wireless card, have voice capability
to make and recieve calls, have a thumb key board, and support AOL
instant messaging.

By mentioning a Sierra Wireless Card, I'm assuming that you're looking
for something that can function both as a cell phone and a PDA.  Here
are a couple devices:
<a href="http://www.palmone.com/us/products/smartphones/treo600/">Handspring
Treo 600</a> - has smartphone capabilities built in, so you don't need
a Wireless card.  The Treo 600 is GSM and world-capable - works on
GSM900, 1800, and 1900.  As of now, there is only the T-Mobile
version, but if you're a T-Mobile customer for more than 90 days, they
will unlock your phone for you, leaving you free to use whatever
provider you want.  Also, I'm sure that you can find an unlocked Treo
600 somewhere like Ebay fairly easily as well.
Palm also still sells the <a
href="http://www.palmone.com/us/products/smartphones/treo270-300/">Handspring
Treo 300</a> as well - same concept, just bigger and with a (probably)
more comfortable thumb keyboard.
If you are an AT&T Wireless customer (my condolences to you if you
are, I've been shafted by them) and you prefer speaking through a
hands-free unit, the <a
href="http://www.palmone.com/us/products/handhelds/tungsten-w/">Palm
Tungsten W</a> might be more up your alley.  It has a more traditional
PDA form factor, and also has an SD/MMC expansion slot for extra
memory, which the Treos lack.  This PDA can accomodate an SD 802.11b
card - I forget who makes them, but they do exist.

Hope that helps,
szehui-ga

This is for szehui-ga who commented on the question. I am the one who
asked the question. The problem with Palm based solutions is that they
do not have support for AOL instant messenger.

Palm doesnt support AOL, but AOL do have a suite of programs for the
Palm which includes AIM etc. etc.

On the Palm, you can also access the AOL Instant Messenger network
with Verichat or Chatter.