The iPhone will continue to gain market share, I can't say for sure if it will become the dominant business phone but I wouldn't bet against Apple. Actually, I see RIM losing market share as opposed to Apple taking it from them. IMO this is RIM's game to lose not the other way around.
I say that because RIM seems to be comfortable where they are, they are introducing more phones but they are not improving on core functionality at a fast enough rate. The BES and BIS phones still suffer from old school file size and download limits, a 5MB limit in 2009 is laughable when people are paying for "unlimited" data plans.
I'll continue to beat my favorite dead horse and say that RIM's failure to support activesync for Enterprise MS Exchange and Lotus notes messaging systems will continue to be a major fail in the business sector. Customers are realizing that they don't need to pay for a BES server and hire a dedicated BES admin just to be able to sync their entire messaging system. RIM is supposed to be the king of messaging but unless you invest thousands of dollars for a BES implementation, you can't sync subfolders, contacts or your calendar. You can't even browse your enterprise address book on BIS, this is laughable when everyone else gives you all of these without the BES tax. As far as security, you can't even remote wipe a BIS blackberry if it's stolen, where is the security in that? A BB on BES is about as secure as you can get and I think RIM will continue to own the sectors where security is more important than functionality, but there are hundred of companies, mine included that are realizing that you just don't get a great ROI with RIM products.
Apple needs to do a few things if they want to take a bigger piece in this market. They need to come out with a device that has a physical keyboard. I know they haven't done it because they can make one device for every market and just change the language via software which keeps their cost down, but not everyone wants a touchscreen.
The lack of true multitasking on what is essentially a tiny UNIX box, the original multitasking OS is a major fail on Apple's part. This could be easily addressed as I multitask daily on my jailbroken iPhone and see absolutely no performance issues with it.
Finally, Apple must stop spoonfeeding us technology, copy/paste while great on the iPhone should have been available on the 1G. Saving email attachments to the phone, being able to browse/open files from the filesystem and downloading from the Safari browser are still not available out of the box. I have all of these features on my jailbroken phone so I am a little better off, but these are what I would call serious business functions that I shouldn't NEED to jailbreak to get.
Apple is opening a huge data center here in NC, so I think the push email argument will soon go out the window. I think they will be taking a page from RIM's book and running alot of things through their own servers in the near future. They are adding a ton of processing power here.
Sorry for the novel, but since I am a BB and iPhone user, I see how these devices compare every day and IMO Apple has the momentum while RIM is sitting back enjoying their success. Once Apple addresses a few of the major issues and links up with VZW, things are going to get ugly for RIM.