Chatr offers US roaming. Yes. You need to be travelling of course to a place that has network coverage and remember that special roaming fees will apply. Currently, Chatr's rates when inside the US are:
50¢/min - for incoming and outgoing local calls
50¢/min - additional charge for outgoing long distance calls
free - incoming text messages
15¢/message - outgoing text messages
50¢/message + $6/MB - outgoing MMS messages
$6/MB - data usage
Not cheap but that's the cost if you want to keep your phone number. More economical of course is to have an unlocked phone and buy a local SIM when you arrive the US or buy a special roaming SIM and a plan with someone like Roam Mobility.