I am interested to find a lightweight View Source editor that I can
invoke from IE using the View --> Source command, or the right-click
context menu.

Constraints:
:: must have syntax coloring
:: syntax coloring must be configurable
:: must be lightweight -- load as (or nearly as) quickly as notepad
:: must be able to handle spaces in file names*
:: must be able to correctly identify cached browser files as HTML**

* I've used notepad.exe replacement tools in the past that had trouble
recognizing a file was HTML-formed. This seems to be the way in which
IE caches the current file and names it an arbitrary name, like
"index[1]" with no .htm or .html suffix. This rendered the syntax
coloring features of the view source tool impotent. (NoteTab Pro,
TextPad have this problem.)

There are published Windows registry hacks to set any .exe to the be
the default View Source tool; I don't need that extra info.

I would be thrilled to find something like BBEdit for Windows, but
sadly, my search for such an elegant little program has be fruitless.
I currently use Dreamweaver MX 2004 for most HTML development, but
this program is a pig and isn't well-suited to just Viewing Source in
the browser. For one, it fails to identify a cached page with no
.htm/l suffix (so syntax coloring doesn't work for pages named like
"index[1]". Secondly, it's just too slow to load.

Good luck.

-- Font9a

Hello, font9a:
  This question reminds me a problem I had some time ago, so my answer
is based on my own experience, in any case, don't hesitate to request
for any clarification, I'm here to help you until you get what you
need.
  
  My choice to replace the notepad on IE when using the option View
Source..., is HTML-Kit, you can find it here:
  
HTML-kit  
http://www.chami.com/html-kit/    

HTML-kit features
http://www.chami.com/html-kit/features/

You can download it from here:

Download HTML-Kit
http://www.chami.com/html-kit/download/

After you install it and execute it for the first time, you will be
prompted for several questions, one is to make it the default View
Source option for IE, so you don't need to change anything on your
registry.

Regarding your constraints:

:: must have syntax coloring
:: syntax coloring must be configurable
"Real-time syntax highlighter with the ability to customize colors and
font styles to make it easier to read code. Maintain multiple
colorizer views and/or highlight only specific parts of code, such as
text, tags, comments, scripts, strings, ASP, etc. Includes
language-level color coding support for HTML, XHTML, XML, CSS / XLS /
XSLT Style Sheets, JavaScript / JScript / LiveWire, VBScript / Visual
Basic, ASP, PHP, Perl, iHTML, Java, Python, C/C++, .NET C#, Delphi /
Pascal, INI, and IE 5 conditional HTML files with customizable file
extensions. Coloring of JSP blocks is also supported."

:: must be lightweight -- load as (or nearly as) quickly as notepad
-It loads very fast and it's very lightweight, not as fast as notepad,
but very fast.

:: must be able to handle spaces in file names*
:: must be able to correctly identify cached browser files as HTML**
-It works with cached pages without HTML extensions, just try it and tell me

I suggest you to try it, but if in any case, it doesn't fit your
needs, just request for any clarification and I will look for another
solution.

Regards.

Theres a very simple method of just changing the source viewer to any
program you desire, if that's what you're hoping to achieve...

Run REGEDIT, follow the following directions to the proper key.

HKEY_LOCAL_MACHINE 
|- Software 
|- Microsoft 
|- Internet Explorer 
|- View Source Editor 
|- Editor Name (Default) = C:\windows\notepad.exe

If this doesnt exist (but it should) then create the Key "View Source Editor"
Then create a Key within that named "Editor Name". Modify the
"(Default)" value to make it point towards any program on your
computer using "C:\YourProgram.exe" (without the brackets).

I hope this answers your query! if not, then i was too lazy to read
your whole question. Sorry!