Update...
Adafruit makes a "USB Console Cable #954" (a.k.a. "USB-to-TTL Serial Debug Cable") (Newark part 44W3509) that goes for $9.95. In addition to being low-cost, it also brings out the USB's +5V which you can use to power the Netduino Mini. It makes the "pay-to-play" price for the Mini about $40 ($30 for the Mini and $10 for the cable).
I used to use a Parallax "Board of Education" (serial or USB versions), but they're pretty pricey ($70 for the board alone, and $100 if you want to get it with a BASIC Stamp 2 module).
The only issue with the #954 cable is that your firmware needs to use USART0 (instead of the DBGU). I hook it up as follows:
[font="'courier new', courier, monospace;"]AT91SAM7X512 Netduino Mini Adafruit 954[/font]
[font="'courier new', courier, monospace;"]TXD0 PA1(A) TX0 DIP 11 white[/font]
[font="'courier new', courier, monospace;"]RXD0 PA0(A) RX0 DIP 12 green[/font]
[font="'courier new', courier, monospace;"] 5V DIP 21 red[/font]
[font="'courier new', courier, monospace;"] GND DIP 23 black[/font]
[font="arial, helvetica, sans-serif;"]Regards,[/font]
[font="arial, helvetica, sans-serif;"]Tom[/font]