I managed to find some information in a Cisco document:
Embedded Service Engine Overview
Cisco Integrated Services Routers Generation 2 (Cisco ISRs G2) have dual core CPUs on the motherboard. The first core runs Cisco IOS software as Cisco Integrated Services Routers (ISRs) do; the second core, or the Embedded Service Engine, is capable of running Linux-based applications.
The following Cisco ISR G2 platforms support the Embedded Service Engine:
When the Embedded Service Engine is configured on a Cisco ISR G2, the infrastructure required to run an application on the Embedded Service Engine is partitioned between Cisco IOS software and the Embedded Service Engine.
When embedded applications are enabled, the Embedded Service Engine requires a compact flash card in the second compact flash slot.
Note CIMC-E is the only application supported on the Embedded Service Engine in Cisco IOS Release 15.1(4)M.
Thanks for sharing the information with us Alan! I'm sure someone will need to research the topic at some point.
I've personally worked with the Integrated Service Module (ISM-SRE-300-K9) which is a platform that supports a number of applications, including Unity Express (provides advanced autoattendant and voicemail support). Cisco has moved from having specific cards for these applications, to one platform upon which you can load a variety of applications, making them more flexible.
Of course, licensing is a topic you can't avoid with these new engines as they strictly follow Cisco's new licensing policy, so you must have the appropriate licenses purchased, otherwise they won't work!
I think as Chris says they are angling to embed value-added things like VoIP and other lucrative applications onto the platform as well as just having it routing. Regrettably as Chris also says you'll have to have a license for the app as well so it won't be that easy I suspect to devise your own. Bit of a pity, but there you go