theres already a 29 page thread on this.
http://forum.ppcgeeks.com/showthread.php?t=27025
the cap is for PAM (phone as modem) users and data card users.
ive posted 2 or 3 months of my usage on my sierra 597 usb modem. ive gone over 30gb in a month and ive heard nothing of it.
this has nothing to do with your phone plan what so ever unless you pay for phone as modem.