I really appreciate everyone's help in this thread. Your advice has been valuable and very useful.
I've been trying for months to figure out how to come out to my wife. Well...apparently she found a pair of heels....and they weren't hers. She confronted me a few days ago, and it all came out. I definitely didn't want it to go down that way. I wish I had been the one to bring it up, but that's how it happened. And she said it actually helped her to have a couple days before we talked. She spent a few days putting together the clues and so she wasn't blindsided by the conversation.
Thanks to all of you it actually went very well. She said it hurts that I lied, but she understands why I've kept it a secret. And it really helped that I've never talked to anyone else about it.
I won't go into great detail, but I can definitely say that this thread gave me some good tools and helped me to better understand her frame of mind.
The most important thing for me was the understanding that I've had years to come to terms with this. I've also researched the crap out of it. But this is brand new to her, and aside from a very general acceptance of transgender people, she doesn't really know that much about it. For instance, tonight we were talking about living life after transition, and she didn't even know that stealth was a possibility. I can only assume she was imagining me looking as I do now walking around town in a dress. Yikes!
So over the last few days she has asked questions and seems to be opening up...albeit very slowly. She found the shoes and must have assumed (correctly) that I have more clothes, and tonight she actually told me I should keep them in the closet and I shouldn't have to hide it anymore. It was a HUGE gesture on her part, and it felt like a really big step for me...bringing something out into the open that I've worked really hard to hide for 25 years. So tonight I put most of my clothes in the closet.

As I've thought about coming out for the last month or so, one other bit of advice stuck out to me and it came in handy. There's a difference between full disclosure and TMI. And I never really understood quite what that meant until we had "the talk". There was a point in the conversation where I started to explain something that she hadn't even asked about. I thought it was relevant, but her head was spinning and she was barely able to keep up. I should have answered questions and kept my mouth shut otherwise. That's when I realized where that fine line was. Full disclosure means being honest but being mindful of the right timing. I came close to TMI, and I'm glad I backed off. At the heart of it is the idea that this conversation was about her...not about me.
This is a long post...but oh well.
One last thing is that I have noticed that the couples that survive transition seem to usually work together as a team, and to one extent or another the SO is part of the decision-making process. I hope to someday transition, but I've made no decisions and I'm determined to keep her involved. And I think I've got a good shot.
Next step...therapy! And boy do I need it.