Hello Tobias,
I'm facing a problem very similar to yours in which I can't upload sketches to my board.
I have an UNO as well, and I have tried with versions 0022 to 1.0 without success. I am currently running OS X 10.6.8 "Snow Leopard" and also tried on Ubuntu 10.10 but I had the same discouraging result.
This is the error I get when I try to upload any sketch:
Binary sketch size: 1018 bytes (of a 32256 byte maximum)
avrdude: stk500_recv(): programmer is not responding
avrdude: stk500_recv(): programmer is not responding
I have found in my research many posts pointing at this same error but many go unanswered.
As I have read in many articles the problem in some cases resides on the MEGA8U2 chip (the little one in front of the USB metal socket) that came with a buggy firmware in early UNO versions. So I tried this to see if it would solve it:
…but it wouldn't want to enter DFU mode (may be in your case it could work) even though I tried alternative and derivative techniques. Speaking about me, the problem started when I loaded code that communicated values from the accelerometer inside a Wii controller via I2C to the computer and this caused the serial interface in the arduino to enter an endless loop, ergo, preventing it from uploading new sketches.
Perhaps your PIR sensor was sending data to the Serial Monitor in the arduino environment and that triggered the bug. (Some time ago I used one of those with a healthy arduino to receive infrared control codes and visualize them on a serial monitor, so that's my guess on a possible root for your problem)
I even have attempted burning a nice clean bootloader via a working board loaded with the ArduinoISP sketch but again no success. 
Later I realised I was not hitting the right spot with this move, since the processor (the big ATMEGA328P chip) already has a working bootloader then the offending system is the USB interface itself.
I guess our last hope is to find some computer with Win-XP and see if once there it decides to work. After all, they are the most common in our country so I can't see why not give it a chance. By the way, it is actually plausible that it'll make it on that operating system; I have read that Mac OS X and other *NIX systems handle the Arduino resets differently (including your OS version leopard). I need my Arduino working too since neither I can afford a new one.
I hope you luck
and keep us updated if you find any solution from your part.
P.S. at the moment I'm looking for ways to program the MEGA8U2 chip directly from the empty ICSP headers that are close to it on my board and see if it solves this issue once for all.