When I first noticed it, it was primarily on flights out of Denver, so I just thought it was a problem with the supplier there or something. Then, it started everywhere.

How often does United scrub out and clean the beverage carts and the plastic trays where the ice is kept?

I always take a bottle of ice onboard and usually just get cups of water from the FAs. Also, if getting a soda, the smell/taste of the bad ice is masked to some extent, so usually drinking a Pepsi with it is acceptable. Once or twice, though, the taste even overwhelmed a Pepsi.

As far as not taking ice, I'm not much for drinking room temperature sodas, like most other Americans. It is a problem, though, and avoiding it does not solve it. Since it is so widespread, I don't think its the ice vendors at every United station, I would look to what's happening onboard. If they're not cleaning the carts regularly or something, that could be the cause. If one dry ice bag ruptures or leaks, it could contaminate that cart for weeks. I don't believe it's a health problem at all, just a taste/smell issue. Basically, a quality issue.