This is awesome. It worked on my Trixbox 2.8.0.3 on Asterisk 1.6.

First I got my phones working internally and talking to each other using various guides online. Then signed up with Broadvoice.

Then added your stuff:

Outbound Caller ID...I used my phone number.
Channels and dial rules...I removed the < and the >.
Gave the trunk a name ("Broadvoice")
authname...my phone number (e.g., "2524322334"--hopefully nobody out there has that phone number)
fromuser...my phone number
secret...the weird random key they sent me
username...my phone number
Put "from-pstn" in the user context field instead of putting it in the User Details text area as in your example.

Under user details...
authname=phone number
context=from-pstn...dropped this line and put "from-pstn" in the user context field instead.
fromuser=phone number
secret=that weird random key again
username=phone number

Register string:
2524322334@sip.broadvoice.com:34lksxj94u7s:2524322334@sip.broadvoice.com

...where "34lksxj94u7s" is the weird random key used above.

Then I created a new outbound route with this for the dial patterns:

011.
1+NXXNXXXXXX
1800NXXXXXX
1866NXXXXXX
1877NXXXXXX
1888NXXXXXX
1NXXNXXXXXX
NXX+NXXXXXX
NXXNXXXXXX

and picked the Broadvoice trunk. Then I moved my new outgoing route above the default dial 9 dial pattern in the route order on the right. I guess you could just use the dial 9 pattern, but I don't know how to do that yet.

I haven't set up incoming calls yet, but the outgoing calls are working great.

Cool! Thanks!