I've noticed that sometimes when I 'svn diff' my code before checking it in, indentation has been changed locally on lines in files I've opened but not edited, or in lines of files where I've made changes to other lines.

It's not whole files, either. I'll find maybe a few lines in a function near the one I was editing will have indentation changes. Here's an example:



I didn't edit that function, but I edited one elsewhere in the file and saved it.

Any idea why this might be happening?

Seems you have Optimal Fill turned on and when you saved the file some spaces were replaced with tabs giving the same indent according to "Spaces in Tabs" settings. SVN seems to have different setting for Spaces in Tabs. I'd recommend to turn Optimal Fill off.