Heh, having my wife play with my prostate is a good thing, especially when combined with a blowjob or handjob. Hobo is right though, make sure you're clean beforehand so that your SO doesn't encounter any "suprises".
Serge, what you're thinking of is called "sounding", which is also a whole lot of fun and can be combined with prostate stimulation for a unique and very cool feeling. You can buy sounding kits online form medical supply places.