When would I need to re buy the OS

After a motherboard replacement, there should be no problem with the OS. If you need to replace a harddrive, you will need windows software.

I think the question is whether replacing the motherboard requires a new OS license.  

Assuming the copy of Windows is a Dell OEM license, not a retail license, then the OEM Licensing page HERE explains it this way - basically an exact replacement is OK, but an upgrade or change is not:

Generally, an end user can upgrade or replace all of the hardware components on a computer—except the motherboard—and still retain the license for the original Microsoft OEM operating system software. If the motherboard is upgraded or replaced for reasons other than a defect, then a new computer has been created. Microsoft OEM operating system software cannot be transferred to the new computer, and the license of new operating system software is required. If the motherboard is replaced because it is defective, you do not need to acquire a new operating system license for the PC as long as the replacement motherboard is the same make/model or the same manufacturer's replacement/equivalent, as defined by the manufacturer's warranty.

The only time you'd need to "re-buy" the OS disc is when your warranty has expired. Sometimes tech support even makes exceptions. And you can even request free discs online. http://xps-resource.blogspot.com/2010/01/dell-backup-discs-for-free.html or go to support.dell.com/xpscd.

But if you were asking if changing your motherboard to a different model, or if you we're to upgrade the motherboard using a motherboard not from dell, then yes you would need to buy a new OS. The service tag for your computer is embeded in the BIOS of the motherboard. It would be as if you were installing windows to a different computer.

Thanks All for your replies :)

What if the mobo swap was like for like ?

This statement is absolutely wrong "The only time you'd need to "re-buy" the OS disc is when your warranty has expired."