'Standalone' TikZ pictures

You can use the standalone class for this. In v0.x it used preview internally, but for v1.x it also has an alternative crop option, which works similar to the preview option/package, but avoids its issues with XeTeX.

There is now (v1.0) also a tikz class option which turns any (outer) tikzpicture into a single tight page. This avoids issues with trailing implicit paragraphs. In addition it automatically loads the tikz package.

% tikzpic.tex
\documentclass[crop,tikz]{standalone}% 'crop' is the default for v1.0, before it was 'preview'
%\usetikzlibrary{...}% tikz package already loaded by 'tikz' option
\begin{document}
\begin{tikzpicture}
  \draw (0,0) -- (10,10); % ...
\end{tikzpicture}
\end{document}

Then compile it as usual with pdflatex or xelatex etc.

To include this tikzpicture into a main document load the standalone package there with the option mode=buildnew. Then use \includestandalone[<options>]{<filename>} instead of \includegraphics. This will compile all includes standalone files automatically as graphics and build these graphics if the source file is newer than the existing graphics file. This needs -shell-escape to be enabled to allow the main LaTeX run to call further LaTeX compilers. See the standalone manual for more details.

% main.tex
% compile with `pdflatex -shell-escape main` or `xelatex  -shell-escape main`
\documentclass{article}
\usepackage[mode=buildnew]{standalone}% requires -shell-escape
\usepackage{tikz}
%\usetikzlibrary{...}
\begin{document}
Lorem ipsum ...

\includestandalone[width=.8\textwidth]{tikzpic}

Lorem ipsum ...
\end{document}

I ended up using the external library, as diabonas suggested.

Here's an example with XeTeX:

\documentclass{article}

\usepackage{fontspec}
    \setmainfont[Mapping=tex-text]{Linux Libertine O}

\usepackage{tikz}
    \definecolor{blue}{cmyk}{1,1,0,0.07}

\usetikzlibrary{external}
\tikzexternalize
\tikzset{external/system call={xelatex \tikzexternalcheckshellescape -halt-on-error -interaction=batchmode -jobname "\image" "\texsource"}}
\tikzset{external/force remake=true} 

\begin{document}

\tikzstyle{place}=[circle,draw=blue,fill=blue!20,line width=2pt]

\begin{tikzpicture}
    \node at (0,0) [place] {Ti\textit{k}Z};
\end{tikzpicture}

\end{document}

One has to compile using -shellescape.

A minor inconvenience is, that the appropriately cropped file is named <filename>-figure0.

Perhaps a combination of standard article class and pdfcrop or the cropping facilities of adjustbox would be a work around.

So, you make the picture in a separate file with article class, for instance. Then you use clipbox from the adjustbox package to clip the image included via \includegraphics to the part you actually want...