The dates and times listed on the forum categories should serve as a pretty specific indicator whether a new post has occured recently. If you have been sitting at your computer and get logged off, well you know that anything within at least the last 15 minutes, you haven't read. Shouldn't be that difficult to figure out. Maybe not so easy as a nice colored icon, but still... :)

As for the icon indicator. How about Gray - read, Blue unread, Light Blue - new posts today. Where "Gray" means anything older then a day AND you've logged in since then. "Blue" means it was posted since your last login (regardless if it is a day or older), and "light Blue" is anything posted today, even if you have logged in previously.