So, to follow up...
I got a replacement Trophy yesterday, and it came with 7720 and the radio update already installed... and the VM indcator worked! So, since I had a working and up-to-date (as far as Verizon is concerned) phone, I felt less scared messing with my old phone. While I didn't brick it or anything, my test results are inconclusive and now I'm really confused:
To start my old phone started with build 8107 and the radio update installed, and the MWI (message waiting indicator) did NOT work.
1. I thought I needed to downgrade my boot loader, so I installed a pre-mango ROM from this thread (
[ROM]Spark Shipped ROM Collection - xda-developers). This also downgraded the radio to a much lower version.
As soon as I did this, the MWI started working again. This was encouraging and is 100% repeatable. Reverting to this version fixed the MWI issue.
2. Let Zune upgrade everything to its current version (build 7720 with the radio update). The MWI stopped working.
3. Revert back to the pre-mango build to get it working again, then try manually updating with a 2.01 ROM of build 7720 that does NOT contain the radio update. Theoretically this is how the phone was shipped to me,
but the MWI no longer works!! I can't figure out why, unless something is wrong with the ROM, which I doubt very much. I tried this about 4 times to make sure I was doing everything right, and it still didn't work. I'm baffled.
I have a couple more days before I have to return this phone to Verizon, so I'll play around this weekend if I can. I recommend downloading the "1.03" ROM and flashing to that, see if it fixes your problem, and if it does, try the 2.01 ROM without the radio update, the one WITH the update, or maybe updating through Zune.