Guide colors other than the default value corrupt the SVG file; I found the following string in the svg inkscape:color="rgb:ÿ," and corrected it manually to inkscape:color="#ff0000" which restored the svg file. Inkscape 0.48+devel r10414 on WXP. Bug (?) reported on launchpad.
I have turned autosave on now, and I will stick to blue guides ...
HTH

rosros wrote: Inkscape 0.48+devel r10414 on WXP. Bug (?) reported on launchpad.
I have turned autosave on now, and I will stick to blue guides ...

Please keep in mind that you use an unstable (and outdated, in the meantime) development build - while the bug is a bug (and confirmed already), it only occurs in development builds >= r10315 and not in the current stable release (which does not support colored guides).

Since only a small number of users might be affected (development builds are not recommended for productive use) - your tip does not really fit into the 'Tricks & Tutorials' section (IMHO). It would be better moved to 'Discuss Software Issues'.