802.11x is significantly more complicated than Zigbee, and the TCP/IP stack you need to make it work on top of all that is similarly complex. If you purchase something like the Lantronix WiPort or Digi Connect WiMe to make 802.11x networking as easy as communicating over a serial port, you're paying for a lot of stuff (there's a whole ARM based server in those modules!) that you don't need if you're willing to do a lot of RF hardware design and software integration.
If you're interested in taking that path, you should get some background understanding of the IEEE 802.11x network architecture. This is one of just a couple IEEE standards available for free through the IEEE-Get program.
Once you've got an overview of the networking system, look up the Maxim MAX283X line of transcievers. From the datasheet,
The fully integrated transceivers
include a receive path, transmit path,
voltage-controlled oscillator (VCO),
sigma-delta fractional-N synthesizer,
crystal oscillator, RSSI, PA power
detector (MAX2831), temperature
sensor, Rx and Tx I/Q error-detection
circuitry, basebandcontrol interface
and linear power amplifier (MAX2831).
The only additional components
required to implement a complete radio
front-end solution are a crystal, a
pair of baluns, a BPF, a switch, and a
small number of passive components
(RCs, no inductors required).
That's about as close as you'll get to a generic 2.4GHz transceiver that can be sanely integrated into an 802.11x network.
They cost about $5 in small quatities at the moment. As you requested, these chips only implement the PHY layer of the protocol. You still need to handle the data link layer (MAC and LLC), network layer, and transport layer on top of that before you can begin communicating at the application layer level.
There are no guarantees, not even for the 68HC11. Ask your supplier what the policy on obsoleting parts is. Usually they send a notification with a last buy date. You'll have to buy sufficient parts to cover the remaining production years, or at least until you have a redesign ready. Depending on your relationship with your manufacturer (read: how many parts you purchase per year) you may get an early warning.
On one occasion, for a custom IC the manufacturer didn't discontinue the part, but raised the price to such ridiculous levels that we decided ourselves to stop production. This was an IC produced with an older process which didn't have much production anymore.
Like Olin says, look out for second sources. If your supplier discontinues a part you may still find it at other manufacturers. But scrutinize datasheets. Sometimes second sources aren't exact copies, and the details may need a engineering changes in your design. If you're lucky this is just a resistor value, if you're out of luck this could be an extra resistor.
Also, changing manufacturers may also imply a different price, and a (much) higher price for the part may be a reason for a redesign, especially if you're running large production.
edit
Mike mentions the Flash memory market as notorious for its volatile availability. This is probably due to continuous advances in the field, especially in memory size.
Also expect short life times for emerging technologies, like OLED. I've had OLED modules becoming obsolete before we finished our design!
Further reading
NXP sample product discontinuation notice
Best Answer
Longevity.
Examples:
It is especially important on digital components. You can exchange resistors and capacitors without problems, but digital parts require design and qualification.
Often a last-buy date is given. Before which you have to order your last batch. Keep an eye out for, or register to notifications of Product Change Notes (PCN). These will tell you when and how parts change. (eg: different plastics, different country of origin, new process)
Recently many semiconductor companies have merged and split. (atmel, nexperia, microchip, freescale) Often they will shuffle their assets, and those are production facilities. Expect PCN's after merges and splits.