No need to cancel if your trip is in a month and assuming you are staying in central BKK. We are currently in Chiang Mai and the news here is the worst of the flooding will hit this weekend. After that they expect it to recede and slowly get back to normal. BKK is a huge city and most of the flooding is in the suburbs and some of the areas close to the river.
Though they will be working VERY hard to get the city 100% back to normal and it will take far less than a month for that.
Come on over and enjoy Thailand and don't cancel. almost 7 years ago