There are not any 3G routers with an expresscard slot yet. Watch for those, in Q1- 2008 at the earliest.
However, if you have the 597E -- the TopGlobal MB6000 and MB6800 will work, but you'll need the novatel branded pcmcia-to-expresscard adapter.
if you have the EX720, then it works with those same two TG routers with pcmcia adapter, and it also works with the new CradlePoint CTR350 and PHS300 EVDO Routers, but you'll need a compatible usb-to-expresscard adapter.
Here's a picture of a similar expresscard (verizon v740) in an SIIG usb-to-expresscard adapter, hooked up to the cradlepoint CTR350: