The company I work for is thinking about getting a number of iPads. Normally with computer hardware (laptops/desktops) we plan for replacement every 3 years. Is it reasonable to expect the iPad to have similar service life or is it more likely that we'll be needing to replace these after only 2 years or less?
I think, in the long run, what matters is how well you want to be supported by Apple (especially in a corporate environment). For me personally, if the iPad 3 doesn't have anything all that great, I'll stick with my first-gen iPad if it is working well. But chances will be good that by the time the iPad 3 (or 4) is out, there will also be an increasing number of applications and features in the OS that require a newer iPad, and sooner or later, Apple discontinues support for the older hardware.
Typically (like a good number of companies), this support is extended to current-gen and previous-gen devices. Given how often a new version of iOS is released, in addition to how often the hardware is rev'd, I would suggest that two years is probably the best life-time in the corporate environment. I fear that at three years, you would have some people on unsupported hardware (and out-of-date OSes -- security risk) for at least a period of a few months, if not a year.
Of course, physically, the iPad should still be kicking after three years if it is treated well; battery life might not be quite as good, but it should still be sufficient. The bigger issue is your company's willingness to be out of support on older generations for a period of a few months to a year. (Of course, this is a gamble. Apple could surprise us all and release iOS 6 in a year or two that would work from iPhone 3Gs to iPhone 5 and all the iPads. But it isn't likely.)
It's difficult to say; iPads just haven't been out that long. I suspect that the key issue for iPad replacement will be the battery lifecycle. Any lithium ion battery has an optimum number of charge/recharge cycles. Apple's iPad battery page says:
A properly maintained iPad battery is designed to retain up to 80% of its original capacity at 1000 full charge and discharge cycles. You may choose to replace your battery when it no longer holds sufficient charge to meet your needs.
That same page also says "Charge cycles vary by use, environment, settings, configuration, and many other factors." So as an IT department, I would implement a charge/discharge cycle policy to which your users should adhere, to get maximum time and value from the batteries.
My personal guess is that it will depend entirely on what apps you will be running, and what version of iOS those apps will require. But I would guess that for standard business use (mail, web browsing, some specialized apps), that a 3 year replacement cycle will work fine for iPads.
It could be possible to use them for 3 years, though with Apple's pattern for updates would suggest more for around 2. Usually with the new version of iOS, the current hardware is supported fully, the previous generation of hardware is supported fully (except for minor hardware additions), but then generations before that only get some of the update and it usually slows down the device.
Also, you should also think of iPads/tablets as content-consumption devices. In other words, it will be good at things like browsing the web, reading emails, listening to music, but when it comes down to creating things like documents, writing long emails, recording good sound, the iPad is going to fail miserably compared to a laptop. So before you make any decisions, make sure that it can do everything you are expecting it to do.