Quick fix:
\makeatletter
\let\xeCJK@ignorespaces\relax
\makeatother
In xeCJK 2.4.1 (2011/05/20), you can use:
\usepackage[space]{xeCJK}
\renewcommand\CJKglue{}% get proper linebreaking if spaces are provided
or use \CJKspace to allow spaces between CJK charecters.
Before the new xeCJK updated to CTAN, you can always get the latest code through SVN:
http://code.google.com/p/ctex-kit/source/checkout
The interword space is font dependent, as different fonts require generally different spacing. Moreover it's flexible, to allow justification: it can shrink and stretch.
The values for the default Computer Modern font are
natural width: 3.33 pt
stretch: 1.66pt
shrink: 1.11 pt
The em width in this font is 10pt, so this amounts to saying that the interword space is 1/3 of an em, optimally stretchable up to 1/2 of an em and shrinkable up to 2/9 of an em.
Actually the stretch may go beyond the "optimum", if necessary for justification, but this stretch will contribute to the "badness" computed for the line.
The interword space components for a particular font can be obtained by the following code:
Spacing for the current font:
\the\fontdimen2\font\ plus
\the\fontdimen3\font\ minus
\the\fontdimen4\font
Quad width for the current font: \the\fontdimen6\font
Put the code above after \begin{document}; load in the preamble the necessary packages, for example \usepackage{tgtermes} or similar ones. The code may be also repeated as many times as one wish, inserted in braces and after a font choosing command.
The TeXbook explains also the use of \spaceskip (and \xspaceskip) for changing the interword space from the default established when the font is loaded.
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