I agree with Kay - I only put signatures in the lower right hand corner - usually with a copyright symbol...I think putting them in the center kind of kills the picture...

If they want to steal the pics it isnt hard to photoshop the water mark out - its just a matter of playing around...but not too hard to do if you have a steady hand.

aaand on a side note I am getting ready to blow a chunk of my pre-commissioning loan on a canon 40D with an image stabilized lense...oh gosh I am all twitchy about it