MadMusician, I just had this exact same issue!!!! Not trying to thread hijack, just sharing my experience:
Guy I dated for about three years (last 2 years of high school and the year after that) randomly IMs me one day and says he saw me at the grocery store. Okay, that's nice? I hadn't talked to him since last summer (this was earlier this summer, say mid-July). We talk, catch up, whatever. We both play WoW, we both are in long distance re
lationships (with people we met in WoW, ironically enough).
Then a couple of days later he calls me and asks if I want to go out to get some tea. Uh, ok? He still had my cell number memorized because "some things you just don't forget." Right.
So I go to tea (by the way, it was really really gross. Not the fault of the lady who made it, I just ordered something I didn't like. Ew.) We talk for an hour and a half or so, then go to leave. First he returns to me some blankets of mine that he has had since we broke up. 2 years ago. Okay? And gives me a hug. Awkward.
That night he makes a character in WoW on my server. So he can chat with me. Says he hasn't told his girlfriend that we hung out yet--because she was working all day and they just talked for a minute before she went to sleep. Sounds fishy to me. He asks questions about my rel
ationship, and I gush lots about how wonderful it is. He indicates he's having problems with his GF, I give him lots of helpful tips on how to make things better.
A couple of days later he tells me he gets to go on a spur-of-the-moment trip to visit his GF. And he is COMPLAINING about it because "she'll be working some so I won't see her all the time and I'll be staying with her family and they ask all these questions because they want to get to know their daughter's boyfriend..." Basically I told him that I had zero sympathy, because I would be ecstatic just to spend an hour with my boyfriend, much less a week in his house, even if he wasn't going to be there the entire time.
He comes back and keeps hinting he wants us to hang out again, that he wants to come see my new apartment, etc. He was friends with my coworker in high school and uses that as an excuse--Oh, we could all hang out! It'd be great to see him. I tell him I could just pass along coworker's phone number. He fishes to see if I would be excited to see him again. I play dumb.
Finally I just decided to call him on it--I told him that maybe I was totally wrong, but it seemed like there was some underlying motive. I outlined the things I had noticed, and told him--"I know you. It takes no stretch of the imagination to think that you're sitting there dreaming of a fairy-tale reunion between us." He made some crack about how I must have thought he just wanted to get a little "somethin-somethin" to which I responded: "No, I thought you wanted to get together and start imagining that we would have a rel
ationship again." He says he's a little offended that I'd think such things of him, blah blah, but he quit talking to me.
I think that in situations like that it's best to just be up front. If you sense underlying motives, call them on it. If it seems skeezy, it probably is on some level. Regardless, if YOU don't feel comfortable, the friendship he's wanting to build is automatically not going to work because you're always going to be worrying in the back of your mind.
Good job on making it clear that your rel
ationship is very good, healthy, that you are happy and serious about your BF... if you even hint at unhappiness it gives the guy a chance to get his foot in the door, or at the very least makes him think he might have a chance.
Yeah... if he wants to hang out, make it a double date or a group event. Not just the two of you, not anywhere private, and preferably, not at all.

Your instincts aren't just about this guy and his motives, but probably are alerting you to potential drama/sticky situations that could arise. Mixing old boyfriends and new is generally not a great plan. Even if you are 100% on the up-and-up, you never know when guys will start to feel insecure. Better just to avoid it altogether.