I don't know how you want to use the encoders, but just for pulse counting (no quadrature, etc)look at this:

http://www.8052.com/users/jorlando/mouse.JPG

I needed an encoder also, but very cheap so I used an old mouse. I get the signal directly from the IR receiver and drive the signal with a transistor (the output from IR receivers from the mouses that I opened was around 1.4V) and connected it to P3.4.

The worst part is aligning the wheel encoder with the IR transceiver. I solved this using the mouse assembly itself. I mounted a LEGO gear in the encoder axis, the other at the motor shaft with a rubber belt as transmission:

http://www.8052.com/users/jorlando/encoder.JPG

the mouse worked as an odometer. Noise wasn't a problem, once in a while I had the problem with false reading of pulses (when the window of the encoders is almost aligned, any vibration becomes a pulse) only in the workbench, with the motor running it wasn't an issue.

hth

orlando