I have what looks like the same scenario. I'd like to hear from inetworku on how this might work. From my reading, I can mirror several ports to a monitor port on a switch, but I can't see how to monitor several ports on two switches from a single monitor port. I've been considering the obvious- gunthnp's idea of having the IDS sit on both switches.
I have two C2950T-24's connected together by the two gig ports using etherchannel. 12 ports on each switch are on the vlan that I want to watch. The IDS monitor NIC is currently plugged into port 24 on switch 1. I can see this being tricky since all traffic (including 3 other vlans) goes through the etherchannel port. Gunthnp's suggestion is probably the simplest, but I'd like to know if this is possible.