I am currently in this situation myself. I have just found out that I am pregnant and I am a temp at my current job. They are going to be making it permanent and advertising it very soon. I have decided not to tell them that I am pregnant as I figure that, yes they will need to give me 6 months of maternity leave, but they will get years of quality employment from me when I return

In my last job, (which was for a major bank who prides themselves on being mother friendly -ha) as soon as they found out that I was pregnant they tried to take a new promotion off me and also said I could'nt have it when I returned. I ended up having to involve the union and threaten them with all sorts of things to get them to back off. I didn't end up returning after maternity leave mainly because of the way they continued to treat me after this all happened.

So after that experience I guess you could say once bitten twice shy.