Speculation is rampant around early glimpse of Macworld Expo banners: The banners (normally not seen until after the keynote) read "There's something in the air," which David Morgenstern speculates could refer to the inclusion of mobile WiMax (which he bucks the trend and calls WiMax Mobile, for some reason). I hesitate to stick my neck on the line, but I think it's unlikely Apple would push mobile WiMax at this point. But I've been wrong before.
Back in 2003, I said that Apple wouldn't introduce 802.11g products at the January Macworld Expo event because 802.11g wasn't yet well baked, and Apple wouldn't expose its customers to months of firmware updates and incompatibility issues with other Wi-Fi adapters and base stations. I was wrong! It took eight months of firmware fixes to get 802.11g just right, but we lived through.
In 2007, I thought it unlikely that despite the presence of what seemed like 802.11n chips in many Intel-based Macintoshes dissected by those interested in the innards that Apple would jump the gun on the standard which was still not clearly settled in its direction. Again, I was wrong: Apple pulled the trigger, and announced its 802.11n product and an activator for most Intel Core 2 Duo based Macs that had shipped to that point. A few days later, the IEEE group voted overwhelmingly to approve the draft of 802.11n that settled the issue as it moves to ratification.
Thus my track record is poor on Apple's wireless plans. Nonetheless, I think WiMax isn't in the cards. Rather, it's more likely for Apple to build in HSPA (high-speed packet access), the GSM evolution standard for 3G. I haven't seen this speculated elsewhere, so I may be totally off base, but here's my logic.
Mobile WiMax isn't yet deployed. With 802.11g and 802.11n, you could buy components from Apple and immediately use the higher speed for your own network. With Mobile WiMax, most Mac owners won't be able to access a network, for which they will have to subscribe or pay usage fees, until mid- to late 2008. People generally resent paying for technology they simply cannot use. Apple would also take a margin hit for including the internal adapter, which isn't in wide production yet.
In that light, HSPA is a more reasonable choice with its few hundred Kbps upstream and several hundred Kpbs downstream average performance. AT&T, its iPhone partner, already has HSPA networks deployed in the U.S.; it's determined to roll them out nationally, although its unclear what areas have the slower UMTS standard--faster than EDGE, slower than HSPA--and which have HSPA. (HSPA is often labled as HSDPA for the 3.6 Mbps or 7.2 Mbps raw "downlink" flavor and HSUPA for the 1.9 Mbps or 5.8 Mbps upstream flavors.)
The 3G iPhone will incorporate HSPA, and thus it would make sense for Apple to not be building in technology that's tied to a rival--Sprint Nextel--to its main and exclusive phone carrier partner.
What's more likely, however, is that the "in the air" has something to do with streaming media, a revised Apple TV, and new content than with a new network standard. But given my 0 for 2 record, you might want to take my opinion with a grain of salt.