
Lots of terrible info in this thread. I'll try to make some useful points here.
Anything can be bad for you in the right situation and excess. Oxygen and water, for instance, can be deadly in some circumstances. The key as some have pointed out, is moderation.
General points about Diet Soda:
Can't cause direct weight gain, as many have pointed out, cause it has no calories.
May stimulate the intake of calories leading to secondary weight gain. As far as I know, however, this has never been demonstrated in any noteworthy study.
Could possibly have carcinogenic effects at very high doses (most anything probably could), but there is very little evidence that it really increases risk of cancer. Probably nothing to worry about.
Probably the biggest real medical threat from diet soda is the phosphorous. Taking in the fairly large amounts of phosphorous in pop can affect calcium balance leading to a number of problems. In particular, there is good evidence linking pop to kidney stones.
Overall of course, diet pop is probably not the most healthy thing in the world. It really has little nutritional value, and in high amounts could cause some problems. If used in moderation however, it probably is not a big deal at all.
Whoever mentioned the friend with diabetes should really talk to him and have him discuss it with a doctor or dietician. While diabetes involves high blood glucose, the idea that diabetes = sugar is not really ocrrect. Diabetes is a complex metabolic syndrome that really is associated more with a generally poor diet than with sugar specifically. Eating a lot of junk like McDonald's is probably a lot worse than eating and overall healthy diet and having a pop every once in awhile. Generally the best thing for diabetics is to keep baseline sugar under control and lose weight, which really means keeping calories at a reasonable level, and probably more importantly, exercising.