It is possible to do a full upgrade and have the latest and greatest though the update manager.
That being said, doing this is not the same as a fresh install.
Personally, my OSs tend to have a 3-6 month lifespan before I reinstall them. As I use them, they tend to generate cruft that gets rather annoying. I have learned how to keep the bits that I care about (like, say, music player statistics) through these reinstallations.
In short, no matter what:
Upgrade != Fresh Install
edit more details: The way Ubuntu does it's updates is every 6 months a new release comes out. If your options allow it (and by default i think they do), the update manager will ask if you want to update to the newer release. Otherwise, almost all of the updates are only security/bug fix.
So every six months, you get new goodness, otherwise you are only getting security updates. For example, Firefox 3.5 still is still not in the official Ubuntu repositories, and won't be until you are running Ubuntu 9.10 (Karmic).
Oh, and by the way, if you havent figured out why Ubuntu's versions are so funny, 9.04 = 200**9**, 4th month (ie: April) because that is when it came out. so 9.10 will come out in 10/2009, probably near the end.