O2 of course will be hoping that you reply to those messages via text, in which case they'll be coming out of your regular text bundle. And while they're all for more integration with social networks (Twitter will now sit alongside integration packages from Facebook and Bebo) they're not stupid... you'll be limited to 600 messages a month. That might seem a lot, but with only 20 alerts available a day you'll need to either specify a very short timespan per day to receive the messages, or set it up so you only receive alerts from direct messages if you know your '@' replies are going to overwhelm the limit.
That's certainly true of my account, but this is a good move. When Twitter launched (all those years ago) this option was available to all UK users and proved incredibly useful in the days when flat rate data was uneconomical for networks to provide at a reasonable cost. It was very useful when away at digital conferences where many people had Twitter. In other words, it became a conduit for text messages when American delegates never realised they were using text the way Europeans did.
And now it's back, and just in time for me living on the streets of Edinburgh for a month with just the N97, a video diary camera to live on at the Edinburgh Festival Fringe. Looks like I'll be on an O2 Sim then!
-- Ewan Spence, July 2009.