>Would a high current (20ma/node) shared power/data single
>wire bus be feasible?

Depends on how you want to modulate it. Are you looking at 2 way comms? If 
not then you could transmit out by turning the voltage off in short pulses, 
with a very short pulse for 0 and a slightly less short pulse for 1. If you 
need to have return data, then the current modulation will need to be a 
significant (I would look at around 10%) of the max current to be able to 
detect the change without problems.

>My speed requierments are very low, 500bytes/sec for the whole
>bus would be fine for this application. I'll only need to update
>maybe 100 nodes once per second with say a 1 byte payload,
>4 byte address.

Do you really need a 4 byte address, or is that just trying to be compatible 
to the Dallas 1 wire system? I would have thought that a 1 byte preamble, 1 
byte address, 1 byte payload, 1 byte checksum would be a better scheme.

>I can't say I know much about emf though, and this will be
>potentially thousands of feet worth of twisted pair CAT3 strands.
>Would I be correct in saying that if I keep my slew rates and
>overall bandwidth down, through low-pass filters, it would help?
>Is this likely to even be an issue?

>From this I take it that you are looking to split Cat 3 cable into twisted 
pairs, and string that around. Personally I would look at stringing around 
full Cat 3 cable so you have 4 twisted pairs, and use 3 pairs for power, and 
1 pair for differential data. I appreciate you are trying to make the nodes 
as small as possible, but I would seriously contemplate using a CAN 
transceiver at each node to drive the cable. This is one additional 8 pin 
SO-8 chip per node, plus a couple of capacitors and maybe a resistor. It 
will take care of emc, shaping and thresholds for you, while allowing you to 
use RS485 style comms. Then do the comms at around 1200 baud standard serial 
should work fine. The 3 pairs for power should handle the 2A you are wanting 
for 100 nodes at 20mA/node without serious voltage drops, but again you may 
need to look at using an LDO or switching regulator at each node, and supply 
the cable with a higher voltage.

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