The power injector that ships with the 3COM 8760 is 10/100.
10/100 power injectors are not compatible with a GigE PD device such as the MSM430.
There is a note to this effect in the release notes / documentation for the MSM430.
The 430/460 is OK with a 10/100 PoE switch, but not injector.
If you want to use a power injector, the product J9407B will work with the MSM430.
--glen