Is it possible to create a document for an ebook-reader in the EPUB-format with LaTeX/TeX? Which extension or program can be used?
The answer looks to be Yes, via some converter, but latex probably isn't the best way.
From http://www.web-books.com/Publishing/epub.htm:
The EPUB format is a standard eBook format recommended by The International Digital Publishing Forum. It is essentially a ZIP format. If you change its extension ".epub" to ".zip", the EPUB file becomes a true ZIP file which can be unzipped.
The unzipped EPUB files may be divided into three categories: XHTML documents, packaging files and container files.
You can convert LaTeX to XHTML and then convert XHTML to EPUB.
As noted by @Alexey Romanov a way is to convert from LaTeX to XHTML then to EPub. The ePub standard does not support MathML, but SVG 1.1 is supported and mathematical equations and graphs should be rendered just fine while preserving searchability and scaling.
http://wiki.mobileread.com/wiki/SVG#General_Notes_on_SVG_Usage_in_ePUB
(Please update if you find the ultimate tool-chain to deal with this easily.)