<p>I've just started working on an <code>ASP.NET</code> project which I hope to open source once it gets to a suitable stage. It's basically going to be a library that can be used by existing websites. My preference is to support <code>ASP.NET 2.0</code> through <code>3.5</code>, but I wondered how many people I would be leaving out by not supporting <code>ASP.NET 1.1</code>? More specifically, how many people are there still using <code>ASP.NET 1.1</code> for whom <code>ASP.NET 2.0/3.5</code> is not an option? If upgrading your server is not an option for you, why not?</p>

Increasingly I think not.
The kind of large rigid organisation currently still clinging to 1.1 (probably because they're only just upgraded to it) is also the kind that's highly unlikely to look at open source solutions.
If I were starting a new ASP.Net project right now I'd stick with .Net 3.5 and probably the new MVC previews.