Lists like itemize act differently if they are used within a minipage environment. Specifically, they don't insert that vertical space before. So, you could use a minipage environment inside the table cell for the itemize list. There's a trick which is more handy - define this command:
\makeatletter
\newcommand*{\compress}{\@minipagetrue}
\makeatother
Now just use \compress before \begin{itemize}. You could even use it within the column definition, if you also load the array package:
\usepackage{array}
...
\begin{tabularx}{\textwidth}{>{\ttfamily}ll>{\compress}X}
Output of your example using this command:

The vertical space before itemize is gone. If you would like to compress the lists even more, have a look at
the paralist package, which offers a compactitem environment for that purpose, or
the enumitem package which allows easy customization of list environments, for example \begin{itemize}[itemsep=0pt], it even provides global configuration commands for lists.
But even with \begin{itemize}[itemsep=0pt,topsep=0pt] I would additionally use \compress.
\\in the normal usecase before the lists? Where can one find information such as\@minipagetrue? Is that listed in some manual somewhere? What other stuff is affected by that special "magic" command? I'm trying to avoid a minipage, because I generally don't know the width of theXcolumn of the table, but I would need to know it for theminipage. – Johannes Schaub - litb Dec 04 '10 at 03:13\begin{itemize}[itemsep=0pt,topsep=0pt]was the first thing I tried. It had a smaller amount of space, but it was still there. I didn't look into why. Do you have any idea why? – TH. Dec 04 '10 at 06:59\addvspace. This command doesn't add vertical space if@minipageis true. This way theminipageenvironment avoids extra vertical space at the top. We do the same here for lists - so\topsepand perhaps other values used at the beginning of the list don't matter any more. – Stefan Kottwitz Dec 04 '10 at 13:54\addvspaceand@minipage. List items then set\@minipageto false using\everypar. – Stefan Kottwitz Dec 04 '10 at 13:58pcolumn? – Mario S. E. Dec 12 '13 at 18:38\@minipagetrue? – aleb Aug 31 '17 at 15:22