Good thread! I think as others have already mentioned it does depend on the individual. For my part, well, I really have to think about this one. I would have to say there is a connection for me.
I'm not really a transvestite, though I can say I have experimented with cross-dressing and related activities in the past. I think they stem from the same root.
By that I mean I have always had a personality that more closely matches feminine stereotypes than masculine ones. Cross-dressing was for me a way to express those feminine aspects of myself more than it ever was a desire to be with other men. I think wearing diapers stems (partially at least as there is the sexual turn-on mixed in there too,) from a subconscious need to express vulnerability and need for attention and love.
By wetting diapers I am expressing in a physical way what I cannot find expression for through verbal or other means, just as wearing clothing for women was a physical expression of aspects of my personality I traditionally haven't had the courage to express in most social settings.
Put another way, "becoming a woman," brought a strong feeling of relief for me because I needed that change of gender association to let myself openly feel emotions such as need for protection, vulnerability, etc. (I'm still not very good at expressing those feelings.) In the same way becoming diapered gives me "permission" to express desires of being "taken care of" by another in the most intimate sense.
Of course I could just be over-analyzing a bit here too, I mean there are times my diaper-wearing is done just for the sexual turn-on (can't say I have any attraction to frilly pink stuff though.)
Hmmm, OK I got a bit long-winded again here didn't I? I tip my hat to all of you who are still reading. š Writing this has made me think a lot, thanks for the thread!