What are the bandwidth requirements of a mining rig? How much bandwidth consumption can I expect per 900 Mhash/s rig (for instance) for both pooled and solo scenarios? I'm looking for a ballpark number for the on going requirements, after the initial blockchain download.
With pooled mining, at 900Mhash/s, you'll need a new work unit every 3 seconds or so. Each work unit requires about 256 bytes out and about 768 bytes back. So that's 700 bits per second out and about 2,000 bits per second back. Essentially, zero.

Solo mining is a bit harder to estimate. You will need to receive and relay all new blocks as they are discovered, you will need to receive all transactions that are relayed by nodes you connect to. You can heavily limit the number of other nodes you are willing to connect to, but this will impact your stale shares level and may reduce your transaction fee income.

I measured the bandwidth usage of the client with 60 connections (you can drop down to 8 if you want) and made sure my sample included discovering a block. The inbound average bandwidth was 12Kbps (1.50 KB/s) and the average outbound bandwidth was 9Kbps (1.04 KB/s). So that should give you an idea what solo mining would be like.

Notice that the bandwidth usage is basically insignificant in both cases.

For multiple rigs, there is no adjustment in the solo mining cases. All the rigs can be configured to query the single client. The client's bandwidth requirements will be the same.

However, there is a change in the pool mining case. When a new block is discovered, each miner will need a new work unit immediately since its existing unit is stale. This happens on average about every 10 minutes. Effectively, increase the bandwidth requirements by .5% for every additional mining rig. Note that if you run more than one instance of the mining program on a rig, count it as more than one rig. (This still assumes the rigs total 900Mhash/s.)