I started aikido with a historical wrist injury from karate, and it hasn't really caused problems...but I had taken a couple of steps to address it before I started aikido.
One of these was taping, including taping when not training. When I first got the injury (which I guess I'd call an inversion sprain? for aikido folks, it was kinda like a sankyo gone really wrong), it slowly improved over the course of about five weeks. Then it stopped improving and continued to be present as a nagging, low-level pain that was present during normal daily activities and that would flare up when the wrist was stressed. Finally I started taping the wrist with standard 1" athletic tape every day when I got up in the morning. I left the tape on all day, regardless of how it felt, regardless of activity. I did that for about three weeks, until I hadn't been feeling any symptoms for about a week, and then stopped taping -- problem solved. My thinking is that the wrist needed the support and protection, because without it the injury was being aggravated by daily activities and prevented from complete healing. As far as how to tape it, I used the book "Athletic Taping and Bracing" by David Perrin (http://www.amazon.com/Athletic-Tapin.../dp/0873225023
), which I've found to be invaluable in understanding and treating various types of injuries.
The other thing I did was aikido hand stretches, which I actually learned quite a few years ago when I was studying jodo. My sensei also had a dan ranking in aikido, and he taught us the ikkyo/nikkyo/kotegaeshi stretches (although I never knew what they were called until years later when I studied aikido). They were immediately of obvious benefit for jodo, and I found myself also doing them during the course of the work day, when I'd be doing a lot of typing. I think that over the years, these stretches (which also strengthen) have really helped with my "hand health). Note, though, that like any other exercise, these are not a "more is better" thing (in number of reps, forcefulness or range of motion), especially with an old injury. I think, though, that when used properly, these exercises can enhance your awareness of the current state of your injury, and that in turn can help you make wise choices about how far you push it.
Anyway, long story short, I didn't find that my old wrist injury caused me any problems when I started aikido, but I had already done some things to remedy it, which you might want to think about too. I second what Joep says: tape it to support and protect it, and also to let your partners know that you have an injury.