Let's break it down:
Why are they leaders?
RIM
(primarily enterprise ) consumer satisfaction pegged at 54%. Apple at 79% (25% higher).
What did Blackberry users like most? Email, (56 % of users noting it as the feature they liked the best). No other feature comes even close in terms of popularity
iPhone users - multiple features of the device, seamless integration of a Phone, iPod and internet browser is the leading feature (36%), touch screen interface (27%), ease of use (16%), Safari (5%).
iPhone with OS 2.0 will get enterprise level push email. And a hell of a lot more. RIM doesn't have iTunes, and is playing catchup with touch screen hardware and software.
As noted, they both have a "super-loyal cadre of users that fervently support their phone brand - and each has extraordinary room to grow."
It depends on which analysis/analyst you read. Others have seen RIM as complacent, lazy, and only market leaders due to lack of competition.
E.g.
here
RIM's latest quarter (from December 07) , 34% of its devices were sold to consumers (1.3 million units). Apple sold 1.1 million iPhones in the comparable period. Were the markets sizes for each comparable?
As the article points out, the smartphone market has been growing in triple digits. year over year. And smart-phones amount to <20% of the overall
U.S. handset market. So what with U.S. catching up with 3G tech, there is growth potential in pure numbers for both, irrespective of percentage market share.
That sentiment is also heard
here on Seeking alpha.
The big problem? SDK and apps. Apple is about to open a 24 pack of whupderriere on RIM.
as noted here
"In light of the Apple development kit, RIM's development environment is "comparatively primitive" and will likely result in nothing more than essential "meat and potatoes" apps for the BlackBerry, Wolf says." (Wolf = Needham & Co. investor group analyst).
Both can grow into each others markets (consumer and business).
RIM is coming out with several new launches. If Apple can convert consumer purchases of iPhones into devices that are also compatible with their work -
wow

That'll be a kick in the nads for RIM.
It'd be interesting to see the numbers on usage of Blackberrys and iPhones - do the majority of users have 1 blackberry and a personal phone? Would enterprise users want both still even if they got a new iPhone or Crackberry?
Would they be happy with both?
Till then, we get push email, seemingly simpler Exchange server support (fantastic for smaller start up businesses I imagine), and a shed load of VC money into apps on both sides. (RIM might be 1UPed very shortly on the actual VC values...)
The problem? 3G iPhone is going to be out 1st. The reviews from the enterprises using iPhone were pretty stunning. Simplicity to use, amazing devices etc etc.
Enterprise is niche in comparison with consumer phones.
We all await the cat fight that will occur. But Apple already has the better looking phone I imagine, the better games player, the better music phone, and the better apps phone. I imagine Apple will have some b0000mtastic Enterprise apps too - Salesforce apps, and more.