Apple's annual developer conference in San Francisco.
iPad 3 Announcement on March 7th with Quad Core and 4G LTE? [Date Confirmed]
Overall, the site seems least certain about LTE's presence in the iPad 3:
4G LTE networking has been another mystery surrounding the iPad 3, with a compatible Qualcomm chipset becoming available, but international LTE coverage is still slim, and in some cases a year or more away. It sounds like Apple has 4G LTE lined up for iPhone 5 this October, but we’re still not certain if the iPad 3 will get it earlier.iMore (then called Tipb) had previously pinpointed the pre-order date for the iPhone 4S.
LTE has been rumored to be included in the iPad 3 in the past. As iMore notes, LTE's rollout has been slow, but many carriers are aggressively rolling out coverage and LTE requirements in 2012. Apple has also been testing LTE in iOS 5 and hiring LTE engineers.
Update: Jim Dalrymple of LoopInsight confirms the date with a simple "Yep". Dalrymple is known to have sources inside Apple.