A number of organizations have recently stopped foisting company-issued laptops on their employees, and instead they give them a laptop stipend. These lucky employees have a certain amount that they can spend on a laptop, and they get to pick whatever they want (subject to a few basic restrictions) as long as they can make it work.
The idea of business users toting MacBooks into the office probably gave heartburn to the IT graybeards at some of these companies, but this is the wave of the future. It's also getting much easier as virtualization and cloud computing--two related trends that make hardware specifics less relevant to the software experience--gain more widespread acceptance. But I'll leave off talking about the software side of the picture for now and focus on hardware.
Right now, the best way to save money on mobiles is by encouraging your users to pick up low-cost netbooks. These sub-$500 machines are typically a bit deficient in the horsepower department, which is why they tend to run the dated but snappy Windows XP OS, instead of the more resource-intensive Vista. But the Windows 7 launch will improve the netbook computing experience a bit for many users, which is why now is the time to bring netbooks into the workplace.
The best way to do this is by offering employees a netbook stipend instead of a laptop stipend. They may not be enthused about the performance, but there is an amazing variety of netbooks in some very attractive form factors coming out right now--these range from thin and light clamshell designs with stylish casings to netbooks with screens that convert into tablets.
The combination of Windows 7 and a great form factor may be enough to generate real excitement for business netbooks among your employees, and save you some money in the process.