Dude I have never carried mobiles and never will that's why I opened this thread but pretty much everyone I know has 2-3 lines.
The trend is to have BB and iPhone too these days.
You know, I never thought about it much until you said it. But you are right. I have two lines myself. I have a work line and a personal line. I have a iPhone 3GS I bought used for my personal line and I just had my work Blackberry replaced with an iPhone 4.
Now admittedly for me it's easy as both phones are on AT&T. So I just keep my personal phone forwarded to my work phone.