My wife & I got back from an 11 day trip to Phuket & Krabi. We were there from 14th May to 25th May, 2011.
We were skeptical of having to stay indoors for most of the trip due to the rainy weather forecast. We were surprised to find mostly hot & sunny days all throughout our trip. If I remember correctly, it was partly cloudy on two days (no or very little rain) and there was only one day with heavy rains. A couple of days it rained in the night, but most of the days it was hot & sunny - perfect weather for beach bumming & island hopping.
Thailand is very beautiful in the rains.Unless you're hell bent on catching some sun and lazing around on the beach, hire a car and go for a long drive if it gets cloudy. The roads and the lush greenery are just brilliant when it's wet!
You can get more information on the current weather in Phuket at the Phuket Weather Blog.