Childhood Milestones: The Solo Playdate

As our kids branch off and make new friends at school, did you have apprehensions about that first playdate at his / her friend's house without your presence?  You know, the one where you just introduced yourself to the child's parent, and then left on an errand for an hour or two?  Tracy has a probing question for you.  She writes:

My son is 3.5 years old. Until he started preschool last fall his "friendships" were with playgroup friends and a couple of children of my friends, basically people I've known for a long time and trust with my child. But now, he's making friends in his school and I'm not sure how to support that while still providing the supervision and protection I think he needs. I would be happy to invite another child over to the house to play, but I am not really ready to have him go somewhere else to play until I get to know the other parents very well. If I feel this way, do other parents? I don't have a problem inviting someone over with the invitation for the parent to come as well should that be what they want. But how can I do this without offending the other parent if/when they reciprocate? Obviously if they invited me to their house I would have no problem, but what if they just invite my son? At what age have people just allowed their child to go to someone's home for a play date without them? Are other parents offended if I ask the questions like does anyone smoke at your house, are there weapons there, etc? I'm willing to admit to being considered overprotective in the eyes of some, but I can't help it. Any guidance from been-there-moms would be very helpful.