How much do you need to concentrate when using rollers? I'm consider getting bicycle rollers for use over winter, and I expect after the initial newness has worn off it might become boring. Can I realistically watch a DVD while using rollers? Or do they take too much concentration, and I should look at getting something like a fluid trainer instead?
Rollers are awesome! At least I think so. I have used both a trainer and rollers and I much prefer using the rollers. Once you are used to using the rollers you will be more than capable of doing things like watching a movie. 

One of the best things about rollers is they magnify issues with your cycling form. Once you have trained enough on the rollers your body will figure it out and it will start to be second nature. It's fairly entertaining to do a youtube search for bicycle roller tricks. I don't suggest doing any of those tricks but worth a few minutes to watch and laugh. 

There are several different kinds of rollers, so I would suggest learning about some of the pros and cons of each discussed a bit in another question => Should I buy a PVC or aluminum roller?