Often, I get companies wishing to develop an app for the corporate sector and they ask what phones they should support. If the company is in anywhere but N America I usually say RIM and Nokia S60. For the US, RIM is the primary device.
InvestorPlace has very recently published some research by ChangeWave on the mobile operating systems used by businesses.
It’s interesting to see that between May and August 2010, iOS gained 1%, RIM lost 3% and Android gained 6%. I am also surprised Windows Mobile is so low as it has been used by enterprise for a long time now due to tight integration into Outlook and excellent device management.
The article doesn’t say what countries the "1,602 corporate IT buyers" were in but I suspect it’s US/ N America because Nokia S60 wasn’t even asked/mentioned. If this survey were asked in Europe I would expect Nokia S60 to be between the RIM and iOS figures.
Also, don’t forget people often use their own phone, or a phone purchased on expenses, for business so the actual mobile OS market share used for (as opposed to provided by) business might be very different.
As with many enterprise (and non-enterprise) projects, I usually recommend a survey of what phones are actually being used by target users, before embarking on new mobile development.