It's a bit more complicated than that. I'm not going to get into the technical details, but the BlackBerry Enterprise Server many large companies use presents a serious hurdle DL would have to overcome. A very large portion of the potential customer base would have this limitation.

Also, iPhone apps usually work on iTouch as well. Combined, they present a very large potential population. In particular if mobile check in continues.

Finally, I don't think majority of black berry's out there offer much in the way of UI where one couldn't just use a mobile web site. The iPhone/iTouch have some interesting UI concepts that couple be put to use with an app.

What you really need apps for are phones that don't have full web browser (the vast majority of phone on the network today) and phones that have touch sceen UIs. (Apple, Android, Latest Black Berry). If DL is seriously about mobile apps they would need to hire one of the two or three companies in the US that actually has the experience to create and port an app to hundreds of phones once. One of which is in MSP and is owned by a major avionics maker.