With everything you do online. At least according to this blog post at ZDNet, which somewhat validates my presentation: The Modern Resume - Building Your Brand Online.
There's not a lot in the blog, other than mentioning there are some new tools that give you the ability to leverage your social profile with business cards, the Facebook vanity URLs, and more, but also new privacy controls on your Facebook account.
There are pretty good controls already on LinkedIn, and I think they're good, though I would like to see a little more granularity for sharing with co-workers/friends and a separate level for recruiters or potential interviewers.
I mention it in the talk, but it's worth repeating. Anything you post online is a part of your profile. None of them will make or break your reputation or your brand, but they add up. You don't want a lot of items that present you in a poor light. You want more things that make you look good, or at least professional for your career.
Manage your brand. It's always been something you should do, and that continues in the digital world. I'm not sure if it's more or less important, but I think you can better manage your impressions in the digital world.