After a little research, my husband found that what we were seeing is called "dezincification" and there was a known problem with some brands of fittings and even some class action lawsuits because the parts can fail. Basically, dezincification happens when water penetrates the surface of the brass and removes zinc, which in turn creates a honeycomb effect on the metal, thus weakening it and eventually causing failure. Great.
We decided that even though it is a major hit to the budget, it would be better to remove all the plumbing lines and replace them and all the fittings. I'd much rather take the budget hit now than have everything fail and flood us out later. We used NIBCO (R) HydraPure (R) bronze fittings because they have the least amount of dezincification we could find. My husband also installed a MANABLOC so that we can turn on/off each line individually. This has been especially helpful as we are replacing all the plumbing so that we could get some of the fixtures back up right away. Going without water is not fun, even when it's only for a day. I would have been a terrible pioneer.
Here are some pictures of what we first noticed on the lines.
Here are some shots of the insides of the pipes. For reference, the pipes are all 1/2".
Here is a shot of some of the new plumbing lines and also the MANABLOC. If you like to watch Mike Holmes on TV, chances are you've heard him talk about the MANABLOC before.
I am extremely thankful that my husband is very talented and is able to do most anything our house requires. Thankful for modern plumbing too, even when things go wrong.
Anyway, if you can afford it, I'd totally go the all-copper route. Ours is copper in the walls and under the sinks with PEX supply lines. If you do have any or all PEX tubing, I'd suggest you check the fittings for dezincification. Don't want any floods happening.