Just stumbled across this nifty nugget:
Leaked AT&T Document Hints of an LTE iPad, iPhone Launch in September!
AT&T long ago announced that their long-term plan is to migrate to LTE, and that their HSPA+ was, in a sense, a stopgap measure. They've already announced their LTE launch cities, and apparently are already working with Apple on prepping an LTE iPad for launch later this year.
That means that both Verizon and AT&T users should expect a 4G iPad in the very near future.