I would think the roomba is a great introductory robot to use. Like mentioned above, all the mechanics and electronics have been done for you, and all you need to do is just provide it with some additional sensors and stuff.
If this were my project & since you would like to use some e-net cameras, I would probably go with just a few cameras and a wireless router. Connected to the wireless router I would plug in and ethernet based microcontroller that could send out serial commands.
There are a few other robotic devices I would also recommend - such as the lego mind-storm, however I think the roomba is going to have a lot of "realestate" for you to use. With a few cameras and a router / a microcontroller, and probably some batteries on there, you will need some hefty power to lug all those things around.
One thing to keep in mind is what you want to do your video processing. Video processing is really interesting. I've done some video stuff myself, and when you delve into it - it gets pretty complicated - but rewarding when you can make it work :). Anyways - keep in mind that with video processing you are going to have a "lag" time from when the camera records things to when you program can see it - to when your algorithm can decide what to do.
In addition to video, I would probably also have a few other sensors on the unit. I would probably use an ultrasonic sensor. They can detect fairly close distances (2-15ft) and would probably be good for use with your project. The roomba has a few more sensors you can take advantage of (touch, step, etc.), but again really any robot kit out there works well.
If you are looking for a super basic way to get a great introductory look into the world of robotics - I'd even go as far to mention a BASIC stamp, but that is very limited and you won't be able to do much more with that than ultra basic (no interrupts, etc.)