There might be a Santa Claus, but regarding the mythical animal known as the universal iPhone: there is none. As noted on Apple's spec page for the iPhone 5, the company will be shipping independent GSM and CDMA models of the new phone to match the LTE frequency bands of carriers worldwide. The iPhone 4S, in contrast, supported all its compatible wireless standards in one device.
There's likely to be some confusion around the iPhone 5 model numbering, since there are three actual hardware configurations but only two model numbers: A1428 and A1429. The singular A1428 model is the AT&T and Canadian GSM phone, with support for UMTS/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz) and LTE on bands 4 and 17. Since this phone lacks any CDMA support, it cannot be used with Verizon or Sprint networks in the USA; it could theoretically be unlocked for 3G service via T-Mobile's seven-year roaming agreement with AT&T.
The A1429 phone has a split personality, with both CDMA and GSM units carrying the same designation; the GSM model, however, does not appear to be intended for sale in the US. It supports LTE in the Far East (Singapore, Hong Kong, South Korea and Japan), Germany, Australia and the UK on bands 1, 3, and 5. The CDMA version of the A1429 works with LTE on Verizon and Sprint's networks, plus the KDDI network in Japan -- LTE bands 1, 3, 5, 13, and 25. The CDMA phone also supports the GSM bands, so it should work as a "worldphone" for international travel even if it doesn't include LTE data support outside the US and Canada.
The upshot of all this model madness: You will not be able to interoperate different iPhone 5 models on different networks in the US. Switching carriers (assuming you managed to get an unlocked device) would be limited to intra-standard swaps, and an AT&T iPhone 5 will never work on Verizon / Sprint, end of story.
[hat tip The Verge]
*Verizon is currently in the process of acquiring AOL, Engadget's parent company. However, Engadget maintains full editorial control, and Verizon will have to pry it from our cold, dead hands.