Hey M. -
How awesome and sweet of you, first of all.
Secondly - I'd definitely make sure you're really really for sure pregnant before you say anything! (duh)
Here are a few ideas that came to mind, just to throw in my two cents;
1. an ultrasound photo and a super crafty, handmade card written "from the baby".... i.e. "Baby ______" and use their intended name or their last name.
2. bake a small cake and use food coloring to dye the cake pink or blue, then ice it over so they have to cut into it to find out - and then of course decorate it with baby-ish themed designs.
3. send flowers to each of their places of work, with a card written from you and "Baby ____" using their last name. Or just one flower delivery straight to their home and plan on being there a little while after you know the delivery was made. Lots of florists are very cooperative with surprises like these.
4. send baby booties and a super crafty, hand made card from you and "Baby ______"
5. start a scrapbook for them. get all of the necessary basics (you can buy kits from Target, its super easy and there's no need to spend a fortune)... anyway, get the basic necessities, and make one page yourself, telling them of the baby. decorate it, include an ultrasound photo and anything else you feel would be sentimental and sweet - that way they can have something to immediately get started on once they know they have a whole pregnancy to go through. It would also make a great baby book once the baby is born - you can always add pages to scrapbooks, so they could continue using it after they bring the baby home.
Best of luck to you guys!