We did change the design of MH fixtures over this issue. Either the fixture is enclosed - or the bulb needs additional protection built-in. This is 'enforced' by having slightly different bases on the bulbs. A 'protected' bulb will fit in any mogul socket, but the sockets in open fixtures will not allow the unprotected bulbs to fully seat.
This all came about because a bunch of teachers at a conference got severe sunburn- caused by the bulb in the fixture over them having lost part of the glass envelope.
HID bulbs - unlike ordinary incandescents - do not require a vacuum to operate. I replaced one perfectly operating mercury vapor fixture that had been lighting up just fine for at least two years- even though the outer bulb was completely absent.
The finer points of HID design aside .... and getting back to CFL's ... I admit that the yellowing and brittleness I ascribe to UV is only a theory of mine. I am not able to actually go out and make measurements.
What also seems clear is that there is no point on any CFL that gets nearly as hot as incandescent it is intended to replace. That, I suspect, is why UL is so comfortable in telling you it's OK to use these bulbe- even though the fixtures were never evaluated for use with CFL's.
