1) Bell Atlantic assures us that, because of the redundancy, we can expect 100% uptime from the OC-12. I would like feedback as to whether this is a realistic portrayal of the SONET environment.

Assuming the primary and backup paths are not in the same cable anywhere (don't laugh, it happens), the chances of the OC-12 itself failing inside Bell land are tiny. (Of course, if your local loop gets assaulted by a backhoe, you're in trouble.)

2) We have the option of using either single-mode or multi-mode fiber OC-3 connections - what factors should be considered in selecting the fiber media type.

Single mode goes farther. Multi-mode is cheaper. If you've got relatively short cables, save some money and use multi-mode. If you're dragging fiber all the way across the building, or between buildings on a campus, use single-mode.

3) Which routers should be used. The options are 3com NB-II's DPE+ (dual CPU), Cisco 7200 series, or Cisco 7500 (each router will have at least two 100Base-T LAN ports). OK, I know the 3com suggestion is a loaded question for this list, but has anybody used the 3com's in this capacity? We are a 3com shop that is considering switching to Cisco - this is a significant decision because switching will require us to continue to maintain the existing 3com environment (~500 routers) and the new Cisco routers.

If you go Cisco, I'd recommend the 7500 series. The 7200 series won't handle two OC-3's and two fast ethers. You'll drop packets.