Lpc2000

Vern,

>I was wondering, on the Olimex LPC2148 demo board, if it could be
> powered from a wall wart and have the USB connected.  I see a diode
> there in the 5V line from the USB, is that enough to make it safe?

I'd say so, as long as your wall wart supplies
voltage as spec'ed.

* 6VAC, giving you about 7.8V DC at Vin
* 9VDC, giving you about 8.3V DC at Vin

For both variants, the "+5V_USB-diode" would not be
conducting due to the cathode, that is being connected 
to Vin, is at a higher potential then the anode, connected
to 5V from USB and the board would be powered from the wall
wart solely.

Even if your wall warts outputs leads to Vin less then that,
it still would be safe. 

There would be a small range of output voltage from your wall
wart (about 4.7V-5.7V DC), where both would be contributing to
the supply of the board. 

Below that range the board would be supplied by USB solely,
due to the rectifier bridge G1, but at no time there would
be flowing any current from one supply into the other.

So, from my understanding of electronics, it should be safe
to connect a wall wart and USB simultaneously.

-uwe