And even if the N900 has a bad USB port design, there are a few things that can be done to either solve it or reduce the flaws. 1. NEVER try to insert the USB cables upside down. If it doesn't go in easily, STOP and look at what you're doing. Even if you haven't filed down the USB cable hooks, it should still insert with little pressure.
2. NEVER drop the phone while charging. I've seen plenty of other phones with their charging ports ripped off the system boards because they were dropped while charging. This is NOT a design issue, it's a user issue.
3. File down the hooks on the USB charger and data cable. This will NOT make the cables any less effective, while significantly reducing the force required to plug and unplug them. It's actually amazing how much less pressure it takes once those hooks are filed completely down. If I were to venture a guess based on how it feels (mind you, my hand is not an accurate measuring device), it's say the force required is about a third.
And now for the slightly paranoid among us (I'm one of them): 4a. Use epoxy to strengthen the USB port. I have a write-up how to do it. If you've taken other phones apart, it's really not difficult and it will give an extra insurance. I even tested the epoxy hold on other PCBs, and it takes much more force to unglue the epoxy than it does to rip off a surface mount component. 4b. You can also extend the contact patches of the USB port and resolder it to the system board. More technical and more dangerous than epoxy, but it will definitely strengthen the USB port. Combine this with epoxy and you're golden.
Even if you only follow steps 1,2 and 3, your N900 should survive daily use for a long time. Besides, keep in mind that the N900 is still a cell phone, and its value drops faster than subprime mortgage investments. Replacing it when (IF) it fails, even outside the warranty period, will not be expensive at all.