One would think so but that doesn't appear to work. Whether you use the options drop down from the upper right of the thread page or the options drop down from the upper right inside a post box, either way you get a green band at top of page with confirmation message that says "Success! We marked this idea as new.". But nothing changes, when you go back to the topic list it's still not in bold again.
If you're on the topic list and use the option to "Mark all as New" - that does work though for turning everything in that forum bold again.
