Can a generic 2.4 GHz transceiver be used on an 802.11x network?
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.