I doubt that Microsoft did anything to get Dell to switch. Its a net 0 for MS get 30,000 Win Phones lose out on 10 to 20 server licences for all the BES servers you would need to run that mess.
I cant believe it has taken so long.
If you want a enterprise phone you only have 2 real options blackberry witch needs at least 1 server $50 to $100 CALs and an admin\s for 30,000 of the things you would need at least 1 sql server with huge disk IO and 10 to 15 servers depending on load and then fail over hardware. Plus SQL CALs, network upgrades, ( nothing kills a network like a BES server ) mail server upgrades, and backups. Granted all that gets you total control over the phone that you wont use 99% of . Its also the most secure option against civilian hackers a government just needs to make a phone call and they have everything Blackberry does more to spy on you then Uncle Sam .
Go windows phones lose some of the control you wont use. Run everything off your exchange and AD that you already have. The only upgrades needed would be some extra ram. Get rid of all your Bes servers your Bes admin\s. Your users get a phone that is at least some what modern and since its .net you can make your own apps with in house developers.
Android has not done anything to get in on this kind of market.
Apple is a enterprise joke its cheaper to just keep the Blackberrys.