I got my batch of 3 brand-new 512M Raspberry Pi-s today (hardware revision "000f" according to /proc/cpuinfo). Every single one of them exhibits the same instability: They reset when I connect certain USB devices. If I have such an offending USB device connected on power-on and leave it in, it will work just fine. Which means that hot-plugging USB devices does not work - a critical bug.
I have a couple of USB devices that I can connect without resetting a running 512M Pi:
* A SanDisk Cruzer Micro 16 GB memory stick.
* A handful of non-descript memory sticks (give-away advertising gifts from various vendors).
* My USB keyboard.
But every single time that I plug any of the following devices into a 512M Pi it resets:
* D-Link N Nano Wi-Fi dongle DWA-131.
* Kingston Data Traveller II, 2 GB memory stick.
* An old IBM 64 MB memory stick.
* A pair of USB-powered loudspeakers (speakers with a mini-jack and an analog built-in amplifier powered by 5V USB).
I suspect that the USB power surge when an offending device is connected and powers up, is causing the Pi to reset. This did not happen on a previous revision of the Pi - I have three older 256M Pi's and they can all have all of the above USB devices hot-plugged without resetting (as you would expect).
The power rail on the 512M Pi's USB port is quite dirty. If I power my USB loudspeakers from a 512M Pi's USB port then I have just as much background noise as I have sound. The noise sounds like power supply switching noise, a 2-4 kHz steady tone with high-frequent chirps as soon as there is activity, e.g. the SD card is active. This leaves the audio output completely unusable. If I do the same setup on a 256M Pi there is still too much background noise for comfort but it is clearly less than on a 512M Pi. If I power the USB loudspeakers from elsewhere, e.g. my laptop, and connect the loudspeakers to the Pi's audio jack then the sound is fine - the noise level is at least low enough for casual listening.
I have two different types of 1000 mA 5V power supplies that I have tested with. There is no difference in behavior. I have even tried to buffer the on-board 5V by connecting a flat-ribbon cable to the P1 connector and then, via the ribbon cable wires, connect a 4700 uF capacitor across 5V and GND (pin 2 -> pin 6). No effect.
I have seen a report on thermal instability with the new 512M Pi's  but that issue is not seen here. I have left one of the 512M Pi's running for 3.5 hours straight without any problems - this one was idling and was not overclocked. I then overclocked it to max "Turbo" mode, re-booted, and started a python script that burns 100% CPU. The /sys/class/thermal/thermal_zone0/temp reported 45 degrees C (45000 raw number) when I started the script and stabilized at 53 degrees C after a minute or so. I left it running for more than 20 minutes straight - no problems.
There has also been a report on a bad USB controller connection . We can rule that out. The issue occurs with specific USB devices and cannot be triggered by mechanically exciting the USB connector. Plus, it is the same pattern on all 3 boards.
Unfortunately this sounds like an RMA on all three boards. Please advise me on how to proceed and when I can expect a new batch of 512M Pi's where the USB interface is stable again.
-- Jan Holst Jensen