In a brief update to the official Guild Wars website, NCsoft (or ArenaNet - actually, who does update the official Guild Wars website? Is it you? TELL ME)... 's announced that you'll be able to purchase additional character slots for your account starting this summer.
Each slot will cost Ł5.99 (or €8.99 in Europedollar) and will enable you to add a character to your account, fairly obviously, with full access to all the other content your other chars' enjoy. More details should appear on the website soon.
Oh, and since we're here, we should point out that Guild Wars Factions - the second in the series - is due out at the end of April.