A new rumor on Tuesday suggests that RIM is currently developing a version of its upcoming BlackBerry PlayBook compatible with AT&T’s 3G cellular data networks. CIO’s Mobile Workhorse blog cites a trusted source in reporting that AT&T’s version of the PlayBook tablet will launch in late March or early April. The same source states that an LTE version will follow soon after the release of the 3G version. Only two BlackBerry PlayBook models have been announced by RIM so far — a Wi-Fi-only version that will launch this quarter and a 4G WiMAX-enabled model that will become available from Sprint some time this summer. RIM is on target to begin taking orders for the Wi-Fi version of its tablet sometime in February according to the report, and orders will be filled sometime in March.