I noticed on my 175w MV bucket light tonight that it was actually flickering (when on full brightness). Now, I don't think the lamp could be going bad (no blackening at all on the arc tube); so I'm guessing it's the ballast that's beginning to go bad - which wouldn't surprise me since this ballast has been going strong for nearly a decade, so I guess it could be on it's last leg. It does buzz quite loud, too. Any thoughts?

my guess would be magnetic saturation.

I've heard ballasts that buzz REALLY loudly, and the lamps work fine, does it do this with all lamps?

a buzzing ballast would be caused by loose laminations in the core,or loose windings on the coil. 

also since the iron in the core is ferro-magnetic the core contracts and expands at line frequency this is known as magneto striction.

i dont know if buzzing really has something to do with the lamp,however i notice that in some flourescent fixtures start to buzz when a bad lamp is continously opperated past its useful life.

I've heard ballasts that buzz REALLY loudly, and the lamps work fine, does it do this with all lamps?

I don't have any other lamps to test it with sadly. Though I'm pretty sure the lamp is fine.

form109, what do you mean by "magnetic saturation"?

magnetic saturation occurs when the level of magnetic flux exceeds the capicity of the ferro-magnetic material,it often causes distortion of the current waveform.

Using a welder's glass, I've looked at MV lamps that were flickering and it seems to result from the arc moving about (or 'dancing') on one or both of the electrodes. From what I have seen, it seems more likely to happen with lamps on underpowered ballasts. Most of those newer cheapo yard-blasters with ballasts such as Caster and other no-names run 175w lamps at around 150w (1200 ma). However, some lamps just seem to flicker randomly no matter what ballast is used. Whatever it is, it doesn't appear detrimental.

Same thing happens to me. Caster ballast + Caster lamp = flickering. Not that much however, but sometimes I think the lamps will shut off... 

magnetic saturation occurs when the level of magnetic flux exceeds the capicity of the ferro-magnetic material,it often causes distortion of the current waveform.

CWA uses magnetic saturation to regulate the current, so it's normal.
But the flickering happen, when the lamp start to rectify - when it carry the current in one direction better then in the other. The cause is usually (partial) loss of emission on one electrode, so bad lamp. But it's still the question, if the lamp was not damaged by ballast failure (shorted cap in CWA,...).