Posted this a few weeks ago on Scan's Today only, it's back on again today.
Your iPad on Three - Pre-loaded with 3GB and valid up to 3 months
They also have 1gb preloaded sims for £3.47.
Currently £20 for the 3gb one on Three (1gb=£10):http://threestore.three.co.uk/PAYG/dealsummary.aspx?offercode=DSLPP508
Free delivery via AVforums if you spend £25, so might only be worth it if buying a few or adding to an existing order.
Of course, you could get a microsim adapter to make it normal size and stick it in your current dongle. Good for those who don't want the contract and just want as a one off - use for a month then throw away. That's my plan anyway using my current 3 dongle.
For those of a DIY persuasion -http://gizmodo.com/5539485/how-to-make-a-diy-micro-sim-to-sim-adapter