Remember that Yahoo! e-mail address you started in high school, but forgot to keep using as you entered the real world? Perhaps there are a lot of people just like you, and it's because of people like you that Yahoo! is running short on great usernames for people who actually want to use the company's services in the here and now. A post on the company's blog has made clear that Yahoo! IDs that are going unused for a year will be freed up for use by a new person.
Here's the crux of it: "We’re freeing up IDs, that have been inactive for at least 12 months, by resetting them and giving them a fresh start. In mid July, anyone can have a shot at scoring the Yahoo! ID they want. In mid August, users who staked a claim on certain IDs can come to Yahoo! to discover which one they got."
If you want to ensure your ID remains yours, just log in with that ID (to any Yahoo! product) by July 15th. Or, today, while you're thinking about it. The ID landgrab returns!