I was just pondering this. It would be OK to put a pre-emergent in your garden, however, nothing will grow for at least 2 months.
I work around a lot of this stuff, so I always err on the cautious side when dealing with pre-emergent and herbicides. Believe it or not, if pre-emergent is not done correctly then a person can really screw up growing patterns for an area. That is why I would not recommend it.
Tilling the garden in the fall and putting some pre-emergent down at that time might be the better solution. i just wouldn't do it in the spring. That is my own personal opinion.