Well, usernames and passwords wouldn’t be too bad, along with forum posts and topics. It’s certainly been done before. Each transfer would be slightly different, but it most certainly can be done. The reason I say it’s a custom job is because for you, you don’t have to do permission masks, etc. Others do, and everyone has a different preference on how they want some IPB data converted over. Your job would be slightly easier.
I’d be careful about doing a ipb->phpbb->EE conversion. The chances of losing data are much higher going that route, I would think (Two lossy transactions is not good).
What you can do is post a job board description; I noticed last week, I think, someone else was bidding out an IPB to EE conversion as well. I’m not sure what the result was, but I imagine it was bid on.
What version of IPB? The earlier versions are certainly easier to transfer (like IPB 1.3) than something like IPB 2.3.