I'll place a spoiler space here just in case.
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Alright, I cannot reveal just yet how I know this, but I have information on what the "bomb" is that Monica will reveal to Chandler. Apparently in the episode Chandler and Monica are talking about how they finally became involved in London, and Monica recalls how depressed she had been on the trip. Somehow she lets it slip out that her first choice to spend the night was with Joey, and Chandler naturally cannot believe it (I can't believe how stupid she was for letting that info slip out!). Needless to say he is not a happy camper.
Anyway, the issue is resolved near the end of the show when Chandler admits that before he and Monica got together, Rachel would have been his first choice if he was going to have a one night stand.
All in all, I was hoping the "bomb" would be something more consequential, but anything of that nature will have to wait until the end of the season.