Apple could be launching multiple versions of the iPad 3 later today with at least two LTE ones, one for Verizon and the other for AT&T with a third one covering CDMA and GSM.
According to The Verge, this could be a very distinct possibility based on the leaks provided by sources familiar with the matter.
Apple is likely to use Qualcomm's Gobi range of baseband chipsets, perhaps the MDM9200 which is a single mode modem with LTE data rates of up to 100Mbps with full backward compatibility and GPS.
As for the GSM/CDMA model, one would expect Apple to stick to the tried and trusted MDM6600 multi mode modem which is featured in the iPhone 4, the iPhone 4S, the iPad (1 & 2) and supports HSPA+ and CDMA with GPS.
The main reason why Apple would choose to have different chipsets would be to cut down costs as Qualcomm would certainly charge more for LTE modems compared to bog standard GSM ones.
ITProPortal.com has a live blog of the event from London, covering the pre-iPad 3 buzz happening in the hours prior to the launch, rumours and the launch itself on Wednesday 7th. We suggest that you follow and bookmark this real-time live blog thread.