Yes they can
The chips in the ppc can snif and inject the datas, there is enough "how to" and softs to do that, look on google but as you said, you need much more power to decrypt a wep key that it is not practicable.
OK, send the data to another computer to decrypt, but so, why not use this computer by default from the beginning ?
There is also a linux bootcd that come with all the tools already packed in.
So yes, you can do it in a lot lot time and it's illegal so I will not give more informations about it but I tried on my own network and it works.
Nobody is perfect m8
Now HTC Topaz (Diamond 2)
Before HTC Touch Pro, Trinity, 9090, S200, S100 and more