We're six days away from the Apple event and carrier details haven't been finalized, which may mean Apple will announce the product without the logical "who and how much" part of the announcement. Network choice would instantly become the elephant in the room.
It sounds odd for Apple to have a major press event without a final carrier deal, but then again, Apple held an event for the iPhone 3GS before MMS messaging and tethering were officially supported by AT&T. And to this day, tethering still isn't supported.
Nevertheless, Apple is in talks with both AT&T and Verizon to support the tablet, according to sources within the companies: One version of the device will run on CDMA networks such as Verizon's, and one will operate on GSM networks like that owned by AT&T.