At least as it applies to Thailand, monsoon season is highly exagerrated in the American imagination. Most parts of Thailand do not get inundated during the rainy season in the same way that bits of India (e.g. last year's floods in Mumbai) and Bangladesh do (e.g. floods practically every year). Parts of central and northern Thailand did experience flooding a couple of months ago at the beginning of the rainy season, but that was a rarity. The coastal bits of Thailand do experience monsoonal rains, and if you're looking for sunshine and long stretches of time on the beach, it's best to avoid areas like Phuket in September. October is pretty much the end of the rainy season (the later in the month, the drier), so even in Phuket or Krabi you'll get lots of sunshine and only brief showers for the most part. Bangkok, which is a good 50 kms inland from the Gulf of Thailand does not experience monsoonal rains in the same way that the beaches and islands do (an urban "heat island" effect may contribute to that, though I'll leave the climatological science to others). Bangkok will be HOT any time of year, especially to someone coming from the cooler climes of Seattle. You'll be grateful for an afternoon shower if you get one-- it will cool the city and clean the air.

Although I haven't been to Bangkok during the height of the Hot Season in April, I'd venture that there's never a bad time to go to Bangkok.