Ian Cant wrote:
Sometimes that is so; what if you install one because
you want to operate in Class A or B airspace [as opposed
to being more visible in uncontrolled airspace]?

Assuming you are talking about the US, I've never seen any explicit
requirement for IFR transponder certification for entry into Class B
airspace (assuming VFR operation). As for Class A (outside of a wave
window) you would be operating IFR, so I would guess IFR certification
would be a requirement, as well as the sensible thing to do.

Marc