Q:

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

I am prototyping a potential future product, and I am struggling to get my head around the complexities of using Wi-Fi components in it. 
I understand that Wi-Fi (or 802.11x) is primarily just a 2.4 GHz signal modulated in a specific way, as is Bluetooth and ZigBee.  Is it possible therefore to just use a basic 2.4 GHz transceiver in my project and configure it for the 802.11 protocol rather than having to use some expensive branded one that comes pre determined?  Or is it not as simple as that? 
I presume in industry, when doing design for large production runs, something similar takes place?  Or do they indeed have to use a transceiver that is preconfigured to be Wi-Fi?  All the pre-configured components I have found so far seem quite expensive (even when purchased in volume) to make a commercial design viable. 
I have looked at devices such as the Lantronix WiPort and the Roving Network WiFly GSX and comparing them to devices such as the Microchip MRF24J40 - is it possible to use a Microchip MRF24J40 and then configure the rest of the hardware and software to enable the device to work on a Wi-Fi network? 
For further detail, here is what I want to do at the most basic level:
Part 1: I want to make a patch that contains a small buzzer, an accelerometer, a PIC and a Wi-Fi transceiver that can be stuck to an object.  This patch would be able to communicate with "The Cloud" though the users' home Wi-Fi network.  When the accelerometer detects movement, the PIC will send a message, via the Wi-Fi link, to a server in the cloud to register that movement along with a time stamp.  
Part 2: Through a web interface I want to be able to send a message to the patch setting the buzzer to make a sound next time the accelerometer detects a movement. 
I have already made a rudimentary version of the system which works over a 433 MHz RF link and a laptop serial port with a local piece of software running on my laptop.  I know how to do the web coding (PHP and MySQL), but it's the replacing the RF link with the Internet that is causing the issue. 

A:

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. 