That's why unread threads have a dot to the left of the title. But really, the bold vs unbold thing tends to vary by OS. They seem very noticeable in Windows; less so on a Mac because of the different text rendering approach.
Wow. If this was Floris' problem (I don't know, combined with his eyes?) then I surely understand the issue now. This is what it looks on my Win XP (with default font settings, no cleartype etc) and FF: