You can choose to prepend your messages with “[From Identi.ca]” or whatever other phrase you prefer.

Start bridging your updates!

Word of caution: This sends every message to Twitter, including @ replies, which probably won’t make sense to your Twitter followers (since they can’t see the other half of the conversation). An option to exclude @ replies from the bridge is in the works.

My $0.02: Brad is awesome for putting this together, and if this is what’s stopping you from switching to Identi.ca, go for it. However, if you’re really dedicated to changing the microblogging playing field and supporting an open-source, federated alternative, then let your friends on other services know they can find you on Identi.ca, and don’t cross-post.