And along with bearpawz point.

When it does power up, the mouse needs to do several things before it can start sending the 3-byte packets. Check this link, starting at Reset Mode, and also look at the Initialization section.

http://www.computer-engineering.org/ps2mouse/

Also, both the CLK and DATA lines are open-collector, so you shouldn't have any HIGH statements. Only LOW and INPUT.
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