I don't work with apple gear that often, so am unsure of the answer to this. Can you stream different source music to multiple Airport Expresses from the same device (say an iPhone)? Spec'ing Sonos on a basic job in which customer stated he likes using AirPlay. Wasn't sure what is possible as far as multiple streams.

No, an iDevice can only stream one source at a time.

Does the same hold true for a jailbroken iDevice?

Yes you can only airplay from one iOS device to one Airplay receiver. You can stream to multipe Airplay receivers from iTunes on a computer. You can also control you itunes library from your iOS device.

With Sonos you can just connect Airplay to one Sonos machine input and then send to multiple sonos  end points with the sonos app. What are you trying to accomplish?

I just did this on a job. The price difference between as "decent" 2 channel receiver, 2 zone Aton wireless volume system and an connecting the clients Express and putting in two Sonos Connect:Amps connects was minimal. We connected the Express to one of the "Connects" aux inputs. There was also a problem with a receiver not fitting in the small area. No problem to fit the Connects.
Now the customer can turn the system on and control it with out the need to go to the closet and turn on a receiver or volume, etc. With an i-device he can control his i-tunes or use a Sonos stream. The big plus is that the wife can now control the Sonos with her Android. Nice advantage versus just a receiver in the closet. She loved it. If she really wants i-tunes she can go to her laptop and control it.

Why not connect stream the iTunes library through Sonos? She can have full control from the Sonos app.

Trying to accomplish a decent streaming system on an incredibly tight budget. All I'll say is this client is going through a divorce and his money is not really his money anymore. He already has an Airport Express and we will probably end up streaming through that for now. Will be adding an Episode 8 channel amp with episode speakers. Probably in a couple months, we will add the Sonos Connects.

Honestly, I haven't "imported" i-tunes into the Sonos system. I have no clue on the use of it.
The husband actually was against using Sonos because of bad reviews, etc he found on the internet. I explained the use of Apples Remote app for actually controlling his i-tunes music and he felt better about the Sonos system.
How does the "importing" of i-tunes work with Sonos? Does the user have cover art, and actual i-tunes play lists? I really should know how it looks and the use of i-tunes with Sonos.
I think the i-tunes part of Sonos will not get used much once a person sees all the other ways to stream music.

You do not actually import the music to Sonos. It builds the library basically. It will display the music much like on an iPod classic. I do not recall if it will show cover art. The best way to accomplish integrating itunes with Sonos in my opinion is to save all your music on an NAS. You can stream directly off that. Works great.