As of 2011-10-01, the "Mango" update has been issued on a rolling
4 week schedule.  How to cut ahead in the line to get the update
(from Ben):

Plug in the phone
Start zune
At upper right, click on settings
Click phone (left side of horizontal list)
In the list on left, click update
If it offers the update, accept the offer.  If not use this trick:
Click on any other activity in the list.
Click on update again
Wait 1 sec; this is very timing sensitive.
Pull the rj45 Ethernet cable.
Wait 30 sec; eventually it realizes it has no internet connection.
It should offer the update at that point.
Reconnect the rj45
Proceed with update.

We were offered 7.0.7403.0 .  This was kind of small.  Probably the installer.
Then 7.10.7720.68.  This one was big enough to be Mango. 
Downloading and installing it took about an hour.

windowsphone.com = http://www.microsoft.com/windowsphone/en-us/default.aspx
-> HowTo = help pages.  

New in Windows Phone 7.5
    .	Use your own MP3 or WMA file as a ringtone.
    .	List view of voicemail, pick the one to listen to.
    .	Voice controls and response, e.g. read a SMS, compose a reply.
    .	Threaded conversations, spanned over various media e.g. SMS+facebook
    .	E-mail grouped on subject (optional)
    .	Unified e-mail inbox (from multiple accounts)
    .	Contact groups; pin a group to Start, live tile shows status of members.
    .	New ways to post stuff on social networks
    .	Native apps for Twitter and Linkedin.
    .	Improved the "Me" card.
    .	From a Contact card, see history of calls, mail, SMS, chat.
    .	More support for multitasking.  How: long-press Back, shows list of
	5 most recent apps.  From there, flick right to see all apps.
    .	Live tiles (on start page) show more stuff (and eat battery).
    .	Lots of must-have new apps in the market, and better searching.
    .	Calendar subsets; to-do list integrated with calendar.
    .	Search from product barcode.  Voice search.
    .	Local Scout is a search for businesses at your physical location.
    .	Mall floor maps from Bing
    .	Driving directions service
    .	You can attach videos to e-mail or post on Facebook or Windows Live.
    .	Camera settings can be saved for later use.
    .	WiFi access point mode, routed to cell data.
    .	Can connect to "hidden" networks.

Customer complaint:  
    .	No tilde on software keyboard (submitted to suggestion box)

Browsertest checkout:
    .	JPEG image OK
    .	PNG image OK
    .	GIF image OK
    .	PDF document -- Requires Adobe Acrobat Reader which can be, but
	isn't, installed from the market.
    .	WAV audio -- very jerky, i.e. plays a segment, pauses, plays again.
    .	OGG/Vorbis audio -- format not supported.
    .	MP3 audio -- OK, plays in a streaming manner, no pauses.
    .	OGG/Theora video -- format not supported.
    .	Flash video -- format not supported.
    .	Quicktime/RPZA  video -- Starts an app, which says it can't play it.
    .	AVI video -- Starts an app, which says it can't play it.
    .	Java applet -- not executed, alternate text was shown.