I agree with those who say that since you've offered to remove the baby (which I certainly think was a very gracious thing to do-and you'll probably enjoy an adults only night), you should remove the major baby things as well. And you'll appreciate having the extra room for entertaining as well! Don't remove pictures (if you even have any up; my twins are now 1 and we don't have any photos of them up yet), but certainly the playpen and toys. I wouldn't think he playpen would be that difficult to break down-ours isn't, and we also travel with it.
ETA: A very good friend of mine discovered she couldn't have her own kids right around the time I was really starting to show with the twins. For her sake, knowing how tough it was for her, I backed way up and allowed her to call the shots. When we were together, I didn't discuss my pregnancy, and if asked about it by others, I would answer the question, then bean dip. I let her know I was there for her, but didn't want to be in her face about it. Since the twins have been born, when ive invited her to things, I've been ecstatic when she has come, but accepted her declines as well. When we can, we get together without the kids, and we don't talk much about them (which is a decided relief, as a stay at home mom). All that is to say, I probably would have accepted the declining of the invite, but since you've made the step to keep the baby out of the house, you really do need to help her and her husband to be as comfortable as possible.