Sarah,
If you plan on going fulltime after FFS - and you'll probably have to - one month may not be enough time for management to prepare for that. They may feel very rushed to get policies and procedures in place and feel cornered if you already have a fulltime date. Every workplace is different of course, but I figure the more time they have to prepare, the more they'll appreciate it. That doesn't mean you have to come out to employees before FFS of course, so only management would know until shortly before the Big Day - or whatever you and they decide is best.
But simply walking in one day and declaring that you're going to be coming to work as a woman in four weeks... probably not the best idea. I know if I was a manager, I'd be really angry if my employee had known about this for months and months and was only giving me a month's notice to figure out bathroom issues, how to tell the rest of the company, plan for name changes, etc.
Find out everything you can first though about laws in your area which might protect you, as well as your corporate policies for discrimination and all. Get some sort of "package" together now, just in case you're outed before you were ready. Identify who you should approach first... sometimes THEY will help you figure out what's best to do and when.
~Kate~