In the UK, I like Virgin Mobile. £10 for a joining pack, and it comes
with £5 of airtime. 15p/minute for the first 5 minutes of a day,
5p/minute thereafter. 20p/minute to call Australia or the US, 20p
International SMS. Can't speak to coverage; I didn't leave the London
area much when I used it. Coverage map looks ok.

In the US, you're probably best off with T-Mobile. The other two GSM
providers, AT&T Wireless and Cingular, use 1900MHz and 850MHz for
their GSM, where T-Mobile uses only 1900MHz. Thus, you may end up in
an area where they use 850MHz, and unless you have a phone that
supports 850MHz, you'll be out of luck. T-Mobile coverage is decent in
cities, less great in rural areas. Not sure of the starter pack cost;
rates are given in terms of minutes: $10/30 min, $25/100 min, $50/250
min, and $100/665 minutes. Note that in the US, incoming calls also
use up minutes, but there's no surcharge to call US mobile numbers.

To call internationally, you're better off getting a calling
card. T-Mobile international rates are bad ($1.50/minute)

Virgin Mobile UK: http://www.virginmobile.co.uk/
T-Mobile US Prepaid: http://www.t-mobile.com/prepaid

Bob