Converting RGB to CMYK on the Fly

I was wondering if you can help me. I am creating .pdf's on the fly and need to have software that will make a conversion from RGB input to CMYK output. I dont need to have every picture tweeked, but would like something that holds the RGB values to CMYK better then nothing, current solution. Do you have anything or recommend anything that might work for me?
Tom

Don't know if this is any use to you, but Brian Etherington's Rgb2Cmyk freeware utility program, will convert RGB tif files to CMYK. (Does pdf support tif?).
http://www.smokingun.co.uk/download/rgb2cmyk13.zip