I switched from my work permit to my spouse visa because I wanted to change jobs, and as garry says, you can't do that on a work permit. Otherwise I think you're fine.
Oh, Garry, I wonder if you can answer a question for me. I lived in the UK for two years on a work permit, got married, and will have lived here two years on that in April 2006. Will they let me go ahead and go for citizenship as soon as I get ILR, since I have four total? TIA!