A bit of deja vu
Well, 3G was supposed to be a mixture of FDD and TDD, and the European UMTS 3G standard contained a TDD mode called TD-CDMA, which was supposed to run alongside W-CDMA. European (and other) 3G auctions included some spectrum for this mode. I don't know of any operators who actually used it - China Mobile eventually rolled out a network based on a different but possibly in some ways related TDD 3G standard called TD-SCDMA. However, it is easy to forget that a decade ago the focus of 3G was actually on providing more bandwidth and higher quality for voice calls, along with being able to provide video calling. These sorts of uses suit FDD - similar amounts of data on the upload and download, and a desire for very little latency.
Now that we are all using (very likely) asymmetric data services most of the time, TDD does come into its own a little more, as it can allocate different amounts of bandwidth to the upload compared to the download, and the amount allocated to each can be varied on the fly. I think therefore that TDD is going to be a big deal in most places for 4G, particularly given that the Chinese are willing to cooperate more with everyone else in terms of using the same standards this time. Given that the Chinese are definitely using TD-LTE from the start, and Apple desperately wants to launch an iPhone on China Mobile, we are going to get multi-mode handset support from day one, too, so my guess is that using the FDD and TDD modes of LTE side by side is going to become standard practice almost immediately.