I wonder if there are estimates of the Emacs user base size, and how it has changed with the advent of new IDEs such as eclipse etc.
It might be possible to form some objective opinion from the Debian or Ubuntu "popularity contest" numbers. Looking at the "vote" numbers (implying the package was used in the last 30 days; far more interesting than a mere "install" count) for Ubuntu, of the ~1.8 million systems I see ~10K with emacsen-common (corresponding to the ~6K/~3K votes for emacs 23/22). So that's emacs in active use on ~0.5% of Ubuntu systems.
For comparison, I'd read eclipse as getting a comparable ~7K votes. Obvious other things to look for are vim (110K votes; 6%!) gedit (97K) votes, nano (33K votes) and even openoffice.org-writer (134K votes).
I'll leave any dubious extrapolations from this (which is no more than a count of Ubuntu machines which have opted into the popcon system and where emacs has been used in the last month) to a count of the global "emacs user base" to you...
Here's an estimate, the size of Emacs user base has grown slowly over time, and likely continues to grow. As a "market share" it probably had its peak in the late 80's and early 90's when there was active development in both XEmacs and Emacs, and a lack of IDEs in general.
Java came along in the 1990s and when Eclipse came into being in the early 2000s, it undoubtedly took market share for the Java crowd, which is a huge segment of the programmers (see Bubble, Internet). Eclipse is to Java what Emacs is to Lisp.
Similarly, the Windows development environment was introduced in 1997, and given the size of the Windows platform (then and now), it likely took a large chunk of developers from Emacs. Emacs is general/multi-platform, the Windows development environment is custom tailored to developing for Windows.
That said, the number of programmers likely grew like crazy starting in the late 1990s because of the growth of the web, so the number of users for all editors likely has grown in that time.
As far as numbers, like Orbling commented, it's difficult to determine the number of people using Emacs b/c people generally use Emacs from the already installed Linux (or other Unix) installation. And those who use more up-to-date builds generally get them from a sysadmin who installs it. But the number of Eclipse|MSDEV users is likely orders of magnitude greater than the number of Emacs users (just take a look at the download numbers for Eclipse - >1.5M for the latest version.)
caveat: This is all a SWAG.