I appear to be having the same issue. I just ordered a bunch of Raspberry Pi starter packs for myself and colleagues. As far as I know, the others are working are fine. Of the two I kept for myself, one works fine, one has nearly identical symptoms (and has since the first time I powered it up). Because I have two, I have been able to swap all the parts and rule out the USB power supply, the SD card, and my equipment (keyboard, mouse, HDMI connection, ethernet connection). All the components work fine on the one board. With the other board, it frequently powers off after a few minutes, and as Bill notes, the only solution is to remove power. I have tried it with mouse, keyboard, and hdmi (no ethernet) and with ethernet only (ssh in, no mouse, keyboard, hdmi) and I get the same results. I have let the board sit for over 24 hours to see if it is just the fuse, but that doesn't help. I currently have my oscilloscope hooked up to both boards on TP1 and TP2. I see consistent values for both boards for all the scenarios I have tried:
* 4.86-4.93V when running with ethernet only,
* 4.46-4.54V while running with mouse, keyboard, HDMI, & ethernet (same values without ethernet too)
When the flakey board fails, the voltage jumps up to 4.93-5.01V, so I don't believe it is the fuse.
On previous nights, the flakey board has failed even when running with only ethernet (no mouse, keyboard, or HDMI). Tonight, it seems to run okay with just ethernet, but fail within a minute if I hook up mouse, keyboard, and hdmi.
I won't claim the flakey board has heat issues, but it seems to have some sort of problem.