T-Mobile UK and BT OpenZone have inked a deal to for mutual roaming across both networks worldwide: This article is short on details, but the press release from BT is quite informative. T-Mobile gains access to 1,900 BT OpenZone locations in the UK. BT customers can use the 8,000 T-Mobile locations in the UK, Czech Republic, Austria, Germany, and, significantly, the US.
Neither the article nor the press release mention cost: is this a free roam or a "you don't have to create a new account to be billed for additional service at the regular day rate" roam?
Interesting footnote in the press release: Through the Wireless Broadband Alliance, the Wi-Fi customers of BT and T-Mobile USA will have access on either companies' networks in Europe and the USA. This is the first we've heard of the WBA for some time; T-Mobile had earlier agreed to join but without providing many details. In fact, their news page was last updated Dec. 2003.