I have to preface this with the word "assumption" to clarify the quality of my posting :)
However, giving it some thought, I'm certain it would not be an issue to the micro - if both defices 'float' at 5v/3.3v with a weak pullup, it shouldn't harm anything that way, I would think. However, the question is, can the device's output handle the weak 5v on the line? Most likely, I would think that would be yes, but I don't know your device tolerances. Since generally logic devices will pull strongest to ground, which is a common point, I would believe this would be your only concern:
1) would the 3.3v device pull the micro's weak 5v pullup down to 3.3v? Probably not, it'll probably reduce it some, though.
1a) if so, is that a problem? If 3.3 is above the micro's threshold for a high bit, that you're OK either way.
2) would the 5v on the output pin harm the device?

If both those are ok, I think it should work fine. I think. Notice the preface on this post, though! :)