Last night I left my desktop PC's browser open to my gmail account and took my laptop to the library to do some writing. I read some new mail on the powerbook, responded to a few things, then came home and slept. Today I came back to my desk to see gmail running, with all the messages I had read and responded to shown as read and responded to.
Since gmail just refreshes the page in your browser every so often, if you read your mail at one location, other locations are updated automatically. This is something POP3 mail rarely gets right and why I used to use IMAP when I would check mail from multiple machines. It's nice to see a web app get this key feature right from the get-go. If you read email at work and at home, gmail will work perfectly fine, even if you leave your mail windows open in both places.