It's not as big a move as IBM/Freescale to Intel, but it's a shift, nonetheless: Broadcom scored an early trifecta with 802.11g back in late 2002 and early 2003 by signing Apple, Belkin, and Linksys for a round of 802.11g-based products. They also swept in Buffalo and several other firms (notably missing D-Link and NetGear) in that heady run-up to 802.11g ratification.
In the latest Apple products, the first to be based on Intel processors using the Core Duo chips, sources outside of Apple told me that Atheros chips have been incorporated: it's true, but Broadcom hasn't been abandoned. Both Atheros and Broadcom chips are specified in Apple documents and are shown in FCC filings.
It's not odd that with a new system architecture Apple would have reviewed chip suppliers, and they may have chosen to work with both Broadcom and Atheros to have competition for their business. There's a limited number of PCI Express-based Wi-Fi chips, which is what the internal, included AirPort Express hardware uses.
The MacBook Pro (the PowerBook replacement) and the Intel-based iMac support 802.11a for the first time, as well. Apple isn't emphasizing the 802.11a inclusion, and the technical specifications only say "802.11g standard."
Although Steve Jobs declared 802.11a "dead" back in Jan. 2003, it was clear he thought it was a non-starter in the consumer market, and the enterprise was far from a win. In Jan. 2006, 802.11a's place as a larger spectrum swath without legacy slower equipment as a way to run more dense, faster enterprise networks and handle campus-wide VoIP is pretty clear. Apple adding 802.11a lets them sell more easily into enterprises and academia that are adopting 802.11a.
One rumor cited by AppleInsider is that the demonstration of the MacBook Pro's built-in iSight video camera was carried over 802.11a to avoid conflicting with the many ad hoc 802.11b networks running at the keynote venue.