Here's that question

Go...
And get the most fantastic fabulous dress possible...
And if you can get a hot guy to go as your date -- then that's awesome too...
If you miss someone's wedding, it really upsets them. A lot of friendships end because people don't come to the person's wedding. If you really like this friend and want to share in her day, go. If it's someone you don't care if you ever talk to again, then skip it...

Here's some thoughts on the opposite side of the fence here. I think it depends on how recent this breakup is. If it is quite recent and you are still actively grieving, you need to take into account how you will react to seeing him. If you are worried that you will break down and cry or otherwise make a spectacle of yourself, I think it's OK to stay home. ..
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Your friend, if she/he is a true friend, will understand. And who cares about what your ex thinks! You no longer have to worry about what your ex thinks. ..
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If you feel up to going and wearing a nice dress and looking fabulously adjusted, as hugandkiss suggests, by all means, go. But ultimately it's not so important to show that you are OK if you are really not. That part of your life is over. It's OK to just have it be over and stay away from it/him, your ex...

You shoudl definitely go...Look as hot as humanly possible and bring a hot guy friend as your date...If you see him, act casual, and definitely abide by the four rules posted by ShannonMarie...If you drink too much or act like a bitch to his date you are just going to be proving to him you are a mess...He will not regret anything, but maybe be glad for the way things turned out.....
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If you do not go you will end up regretting it...You will have a friend angry at her and your ex will know he won...Plus, you will not be happy while sitting home, will you? So why not go, look great, and let him see what he is missing!..

I think you should go. ..
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It might be a painful experience, but at least you'll feel you didn't back down. And it will be more painful to stay away from your friend's wedding because your ex has that kind of power over you. And everyone will think you're cool for showing up. ..
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Also, there's a chance it won't be as painful to see him as you think. ..
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I've had the experience of seeing an ex that broke my heart and expecting to have my heart ripped out again... and finding to my pleasant surprise that seeing them didn't hurt nearly as much as expected. ..
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Either way, I think it will be more painful to stay away...So, um, enjoy the wedding!..

Speaking of someone who fully understands where you're coming from, I say: Totally go. Odds are, you're going to eventually see this guy again, even if you don't want to. This way, if you know when you'll see him, you can control how you look--and of course you have to look stunning. The best revenge!....
Be careful not to drink too much at that open bar. But maybe drink just enough that you 'll be loosened up if you have to say "hey, what's up" to him. Get your hottest friend who he doesn't know to be your date, for moral support. You can do this! Maybe you'll even find he doesn't look as amazing as you once thought he did. And if you brave this potentially uncomfortable situation, you'll feel better on the other side of it...