I tried sharing an ext3 between Linux and Windows now for some months. For Windows I used the Ext2 Installable File System For Windows (IFS). The use case: my wife wanted her Firefox profile both under Windows and Linux.
I never had any problems with IFS, so I didn't expect any now. However I ended up with lots of errors in the ext3 file system twice. I suspect the little difference in the usage scenario (copying files back and forth between Linux and Windows vs. having an application constantly accessing it's configuration files) as the culprit. Perhaps Firefox had a few files still open or was manipulating them when Windows was shutdown respectively switched off the hard way.
This was especially cumbersome because Ubuntu doesn't seem to expect that fsck might ever ask for user input: the boot screen just gets stuck when this happens and no chance to get to the fsck prompt. Hence I had to boot from a LiveCD and invoke fsck then manually.
So now I try it the other way around: using a Windows file system (NTFS) from Windows and Linux. Let's see how this works out...