i've worked in the graphic art/advertising IT for the last 15 years, and i've seen my share of unnecessary attempts to eliminate a particular platform that always seem to bite people in the ass...
eliminating the mac for windows seems to be a favorite cost reduction exercise, but this usually ends up failing due several reasons:
1) fonts - font name mapping for postscript type 1&3 and truetype fonts have always been different between windows and mac. many creative departments collect hundreds(adobe font collection alone around 3000) means extra work to rework creative files created on a windows version of adobe illustrator/indesign for output. this can equate to additional charges spent at the printer/color separator/publication. opentype should eliminate this issue, but in the meantime, there are thousands of legacy fonts that designer will be reticent to stop using...
2) initial cost - macs have always had a higher initial cost, but over a three year life span, i've seen fewer macs replaced in the same time period than windows systems(system boards, PS, pci cards, etc). and if you stage your system refreshes(refresh 33% you your systems every year) older macs get rolled down from the heaviest power users(retouchers, motion graphics, 3d) to layout artists, and eventually to utility systems. you'll be migrating systems more frequently, but using a software deploy system like casper or filewave can simplify this tremendously.
3) talent - creative talent prefer working on macs, and more importantly the talented ones are very efficient on the mac platform. consistency of keyboard shortcuts between various applications and OS, interapplication communication to allow intelligent drag-and-drop between apps, the mac is still better at this than windows, and hence, mac artists tend to be more efficient then their windows counterparts. i've never been in a creative/studio/production environment(save for 3d) where the bulk of the work isn't done on the mac. for these folks, it a punishment when they "have" to work in windows.
don't fuck with the creative/mac department.
sales will always be kings when it comes to having their way (with management) about staying with the windows platform in order to use the latest and greatest new sales forecasting tools, performance analysis, PIM's etc. don't even bother fighting this battle, you'll lose and make enemies. if you take away their toys, they'll probably buy their own replacements(laptops, software ,whatever) and end up expensing it somehow.
don't fuck with the sales department.
basically, don't fuck with anyone who is in any revenue stream role. non-revenue generating IT guys will typically lose these battles.
focus on areas that you have control without affecting end user productivity. you don't want to manage an exchange server? there are hosted exchange services that could do it cheaper than you could. have you considered a hosted sharepoint service for document sharing and collaborative work? ... all without purchasing hardware and software(and hiring dedicated personnel). trying to reduce purchases of hardware and software? work with your vendors to get favorable pricing by making them a partner. good vendors are more likely to offer better pricing to return customers, versus non-repeat i'm-looking-for-the-cheapest-price customers. i've also found in the past that when working with a CFO whose interest was to keep capex spending at a minimum, leasing equipment with a dollar buyout agreement at lease termination always won out over outright purchasing, even though the end result was the same. and when it comes time to get rid of old equipment, donate it to a charity, and get a tax break by keeping track of its book value and letting the accountant work their magic.