The latest Upgrade to WordPress is 2.9.2. The folks at WP decided to release a quick patch for security reasons as they found out that in 2.9.1, logged in users could peek at trashed posts belonging to other authors. If you have untrusted users signed up on your blog and sensitive posts in the trash then 2.9.2 is a MUST for you.
For everyone else – should you upgrade? As I always say, it’s better to upgrade then to wait and find out that you were hit with a security problem. So upgrade at your earliest convenience, and do it before that little old lady that you thought was no threat, reads all those trashy posts you wrote about her – thinking she would never read them because you threw them in the trash! If you set your Trash functionality to off, (see Take Out The New Years Trash), then don’t worry about it – she will never get to read them anyway.