Is there some way to embed LaTeX source code in a PDF file?

I would recommend the embedfile or the navigator package (the latter has the additional advantage of working together with dvipdfmx, see this question of mine). Both are quite similar in usage:

With the embedfile package:

\documentclass{article}

\usepackage{embedfile}
\embedfile{\jobname.tex}

\begin{document}
The document
\end{document}

With the navigator package:

\documentclass{article}

\usepackage{navigator}
\embeddedfile{sourcecode}{./\jobname.tex}

\begin{document}
The document
\end{document}

You can attach arbitrary files to a PDF document using the attachfile2 package.

The above package works only with pdfTeX and dvips. For a XeTeX/xdvipdfmx solution, see this question.

You can do this in ConTeXt as well. Example:

\setupinteraction [state=start]
\starttext
Foo \attachment[file=attachfile.tex, title=Some file, author=Me]
\stoptext

The file key contains the file name, the title key is used (probably depending on the reader) e.g. for a tool tip (see screenshot). Interaction needs to be activated, otherwise the attachment mechanism don't work.