With all versions of Windows prior to Windows Vista, the release of the retail versions (full and upgrade versions) were usually followed very quickly by the release of the cheaper OEM version, which was simply identical to the full retail version but without the full packaging and had to be purchased with hardware. Many retailers would get around this by selling their OEM versions of Windows together with a power cable, which wasn't breaking any rules set by Microsoft as the cable was a piece of PC hardware, but did bend them a fair bit. Naturally customers wouldn't complain, they got a full, legitimate copy of Windows for much less than they would have paid for the full retail version, and who could blame them.
This arrangement changed in a couple of ways with the release of the OEM versions of Windows Vista, in all it's flavours, and changed to such a degree that it may not be the best decision to make. The first change is that you can now buy OEM versions of Windows Vista without having to buy any PC hardware at the same time, this is even stated in the end-user-license agreement. The second change is much more dramatic. The OEM version of Windows Vista has a subtle change to the product activation found in the retail versions, as well as that found in Windows XP. Windows Vista OEM becomes tied to the computer it is first installed to, and essentially the license and the serial key can now only be legitimately used with this PC. This is fine if you are someone who does not upgrade their PC much, or at all... in fact if you are such a person, you may as well stop reading now as you've nothing to worry about. However, if you do upgrade your PC often, read on...
For the most part, this isn't as dramatic as it may initially seem. Microsoft have in no way made it impossible for you to upgrade your PC without having to purchase another license (or in other words, another OEM copy of Vista), there is quite a degree of flexibility. Memory (RAM), graphics card(s), sound card, hard drives and optical drives can be upgraded, changed, or replaced without issue. It's when you come to upgrade the more critical parts that the issue arises, namely the processor (CPU) and the motherboard. Windows Vista OEM identifies the PC it has been first installed to by the motherboard, which it essentially considers to be "the PC", even though it is only one component. Changing the motherboard for whatever reason and you could suddenly find that your OEM Vista ceases to work properly, as it no longer recognises the PC it is installed to. Getting around this by formatting and re-installing Vista won't work either as you'll get to the point where Windows requires activation and once again it will recognise that it is not on the same PC it was first installed to, so activation will fail. Telephoning Microsoft and explaining this problem will, in many cases, result in you being given an activation code but do it too often and Microsoft will certainly reach the point where they say enough is enough. If not with Vista (since uptake of it was poor and the number of end-users is nothing like that which Microsoft hoped for) then certainly with the new Windows 7.
I will point out here that this scenario doesn't always happen, and sometimes you will be able to install OEM copies of Vista onto your PC after a motherboard and/or CPU upgrade and the new activation will work, but you're taking a chance by assuming you will be among those who will not have to deal with this problem (and they are the minority). If you rarely upgrade, or if you are installing on to a laptop computer, you have nothing to really worry about and can simply purchase an OEM version of Vista, saving yourself a tidy sum by doing so. But if you upgrade frequently, as i do, and particularly if you know motherboard upgrades will feature in your upgrade plans, think carefully before deciding on an OEM version of Windows Vista.
*UPDATE* This OEM limitation also applies exactly the same way to all versions of Windows 7