FYI - the Nomi card is on sale @ Telestial also, without the 180-day time restriction:

https://www.telestial.com/view_product.php?ID=LSIM-GB04

Use code "frommers01" for 15% off. Note: the coupon code won't work if you add the $15 top-up card to your order (which you don't need, you can top up by calling a number on your phone).

I bought one and will report back. Tho beware Telestial: Last time I went to London, in fall of '07, I bought the Passport Plus card from Telestial which would only work internationally and not for local calls, stranding me in a rough neighborhood after midnight w/no ability to call a cab. I really should have called to tell them, come to think...Am probably also going to buy a backup from Carphone Warehouse, if I can find a retailer that'll ship to the USA. Planetomni.com used to sell 'em.