When using sitesummary at a site to track machines, it is possible to get a list of the machine types in use thanks to the DMI information extracted from each machine. The script to do so is included in the sitesummary package, and here is example output from the Skolelinux build servers:
maintainer:~# /usr/lib/sitesummary/hardware-model-summary
vendor count
Dell Computer Corporation 1
PowerEdge 1750 1
IBM 1
eserver xSeries 345 -[8670M1X]- 1
Intel 2
[no-dmi-info] 3
maintainer:~#
The quality of the report depend on the quality of the DMI tables provided in each machine. Here there are Intel machines without model information listed with Intel as vendor and no model, and virtual Xen machines listed as [no-dmi-info]. One can add -l as a command line option to list the individual machines.
A larger list is available from the the city of Narvik, which uses Skolelinux on all their shools and also provide the basic sitesummary report publicly. In their report there are ~1400 machines. I know they use both Ubuntu and Skolelinux on their machines, and as sitesummary is available in both distributions, it is trivial to get all of them to report to the same central collector.