Hmmm... Do you have a laptop or something else you could check wifi availability with? Does anyone else you know or notice get the campus WiFi in the same spots where it doesn't show up for your N900?

My first recommendation would be to check that - if other people are easily detecting the WiFi where you aren't, then it's probably an issue with your device. If not, then it's possible for whatever reason, the campus WiFi is just generally sucking or is down temporarily or some such. From there, I would send an email or make a call or drop by the office of, the department in your university that has to do with WiFi. See if they're having any problems.

If not, and you really want to be sure (though up to you, this might be out-of-the-way) go to the spot you got WiFi during student orientation, make sure you have power-saving off (it's in the connection settings, when you get to the list of saved connections, select the campus WiFi one and press edit, hit next until you get to the last one, and press Advanced. Power saving and transmission power are under the Other tab. Sometimes, in some of the classrooms, I have to take power saving off completely to get the N900 to stay on the WiFi consistently. It's not ideal for battery life, but it should last you through a class, or two on a full charge, depending on what you're doing and what you have running/installed). If, power saving off and in the same area you got WiFi before, you still don't get it, and you already know that the campus WiFi is actually up and running, then you are almost guaranteed that the problem is entirely with your device. As for transmission power, I always use the 100 mw (or whatever the measurement unit is) one, not the 10 mw one. I don't really think it impacts power enough to warrant keeping it lower unless you're guaranteed to be right next to the router.

Now, from there, to tell you the truth, I'm not sure where to proceed. On thing I will say is on my campus, in a lot of places, WiFi is patchy, and in others, it's there but the N900 is somewhat lacking in sheer power to blast through the WiFi noise people's laptops create, so it will not pick up the wifi or it will pick it up, lose it, pick it up, lose it, etc. I've had two classes so far where part of my semester is spent fishing for a good WiFi spot in the classroom.

I suspect something like WiFi Eye would be a useful tool for seeing if there's actual WiFi networks around, and what they are (Though I myself don't use WiFi Eye, so I can't vouch for it's usefulness for this purpose).

I suppose if none of the above helps, and no one else more N900-workings-savvy pipes up with technical advice, you can see if uninstalling apps one by one helps. (By the way, you never need to remember what you have installed - when you go to the app manager's uninstall menu, it will naturally list everything you have installed.) That said, I would start with any network-specific or WiFi-specific programs/apps you have. Mostly a reboot after every uninstall should get you a clean slate. Some apps leave stuff behind when uninstalled, which sometimes still run in the background... if that's the case you usually only find out if you dig up the bug-reports/threads for that app. Up to you if you want to look up the apps you suspect in advance or if you want to uninstall everything one-by-one and then look up possible apps if your problem doesn't go away.

You can also open X-Term and type the word "top" and press enter to see what processes are running and eating up the most memory or CPU at any time and copy those outputs here. Might not answer your problem, but if you do it every once in a while when you are trying to connect to the WiFi, you may be able to find something suspicious that clues us in to what the problem is.

- Edit -

If you're not picking up ANY WiFi networks (including any new Free Public WiFi and hp setup adhocs after you've cleaned out your connections list of them, or any other networks you know exist you may want to check against), it's possible your N900 has it's WiFi modem completely off... I'm not sure why this would happen... Perhaps you (without noticing, or for some other purpose) or some program shut it down. I'd if that's the case, you can try running "ifup wlan0" in X-Term. I'm not sure if you have to be root... if you do, you can easily become root by just typing in "root" or "sudo gainroot" first (assuming you have rootsh installed).

Also, possibly stupid question and you most likely already checked for stuff like this, but just in case: Is there a possibility your N900 got set to Offline Mode? (Offline mode button is accessed by pressing the power key briefly. It's one of the options on the menu that pops out. You may have either pressed it accidentally or something may have set it to offline mode without you noticing.) If offline mode is activated, the button you're looking for would say "Normal Mode". By default the symbol next to that next is five horizontal bars on top of each other - like the signal bars, but a bigger image thereof - but if you're using a custom theme it might use different pictures.