If you want to build one for the experiance that is a great. One of the members here did one using an AVR but it was RS232.
If you are doing it to save money forget it. For $150 you can buy one that is very nice. The software on the PC is very good and it does UART, I2C SPI and Dallas 1 wire post processing.
Yeah, the Saleae logic analyzer is amazing. I tried out their demo interface software and it RAWKZ. Don't own one though...kind of what a USB-CAN bus or an o-scope first. IT's cheap enough so I'll just buy it when I have an immediate use for it.