purchase location

US prices don't usually include sales tax. UK prices usually include VAT.

Add shippings costs and the chance that customs will slap an extra import duty or VAT onto it and it's not worth the bother. I also doubt that the guarantee will be worth anything either...

Furthermore, there could be an issue with mains voltages. The G5 iMacs don't have a universal power supply, the minis may not have one either.

Edit: Plus you get the U.S. keyboard without the £ symbol...

 

Mac portables come with a worldwide warranty so if something goes wrong with one you bought in the US, it will still be fixed under Applecare in the UK.

IIRC, desktops (portable tho the Mac mini is) are only covered by a local warranty so you may have to jump through a few more hoops to get your mini repaired over here. You will have to check out the power supply issue - the power supply is outside the Mini so it may be easier to change.

As Blue Velvet said, bear in mind that you will have to add US sales tax in where appropriate, and figure out how you're going to get it through customs without paying VAT. For the price difference in a mini (as opposed to a Powerbook), I'd probably just bite the bullet and buy it here with the peace of mind that gives.