It will very much depend on the network in question and how draconian (i.e. sensible) they are being with Internet Explorer. IEs plugins have often been a source of major security flaws in the browser so a lot of networks do restrict the ability to install plugins.
When I worked in a large secondary school as an IT technician (equivalent to a High School) we had things locked down pretty tightly. If we needed to install updated versions of Flash or similar we had to go round logging in as Administrators (thankfully we had remote management software to allow partial automation of this) to do it because ordinary users couldn't install plugins to IE.
I would think most corporate networks would restrict the ability to install plugins similarily.
With regards to part 2 of your question no Google are not since they can't, they're basically providing a plugin that basically lets the Chrome rendering and Javascript engine run inside IE. This allows them to use standards compliant HTML5 which IE is very behind on implementing and has only as of a couple of weeks ago actually committed to implementing some of the key multimedia features of the specification which are needed for things like Google Wave and future web apps.
See http://arstechnica.com/microsoft/news/2009/09/ie-program-manager-endorses-html-5-multimedia-tags.ars