It wasn't too long ago that there was no "official" documentation that would suggest that Cisco products worked with Open Source Asterisk.
Now with Small Business Pro there is a sense that Cisco has put some effort in documenting and being more open to their products working with Asterisk.
There is a great deal of documentation that is really useful - in fact more than any other vendor. Documents can be found at https://www.myciscocommunity.com
You can find the following Asterisk documentation:
Asterisk: Configuring Cisco SPA5xx phones with the Web-UI
Asterisk: Zero-Touch Configuring Cisco SPA5xx phones
Configuring the Cisco SPA8800 IP Telephony Gateway in an Asterisk Environment
Interoperability information for Asterisk(R)-based Phone Systems
Asterisk: Configuring the Cisco SPA500S Attendant Console
You don't see this level of documentation from any of the following vendors that benefit from Asterisk.
1. Polycom - very little official documentation - even though many of their products work well with Asterisk - if it wasn't for the great Asterisk community many of their products wouldn't even be used with Asterisk systems
2. Grandstream - Just enough to get things working - very little documentation - but at least they acknowledge Asterisk - guessing that most of the IP phones that Grandstream sells go directly for Asterisk deployments. Grandstream is the equivalent of Dlink in the voip world. They sell lots of shitty product at low prices and available everywhere.
3. Aastra - have done a great deal with their xml and auto provisioning toolkit for systems like trixbox and elastix. No enough documenting if you want to configure things manually.
4. Snom - the stuff works - well documented on their wiki for Asterisk - just wish the basic phones weren't so ugly and expensive. The new 800 series look great though!