A protocol analyzer is a much better tool for I2C/SPI debugging than a generalized logic analyzer and a lot cheaper. We use the Aardvark/Beagle from TotalPhase but even that level of debug isn't very common. I2C and SPI don't seem to be what causes trouble anymore. There was one time there was confusion between a "-A" part and the original release, that caused some consternation but that was a misunderstanding of the part. The protocol analyzer told us that it was working right but didn't tell us we were wrong. ;-)