no if it happens then you must kill it like MTS said. but in 1.3pb4-4a-5 and up you can group any unit with any unit so it may happen less now. Also if someone leaves in MP who owns the bomber you have to kill it or wait for someone else to join for it to be usable.
Yeah, that got annoying in an MPI I was playing once where I was in command,
this guy, I think he was called Zachary, kept joining and leaving, and I think he was keeping out the guy whos place he took,
long story short he kept having the bomber when we needed it to destroy something we couldn't, he wouldn't even speak. (he also decided to destroy a couple of my apcs by shooting them, but thats besides the point)
Maybe for the next patch there should be some programming that hands all your units back to the commander if you leave.