It's either SVG, perfectly valid by the spec (and increasingly working across the growing range of SVG viewers because of it) or text, XML maybe, that just happens to look a lot like SVG (and is guessed into an image by some viewers). right?
Therefore a whole lot of SVG out there doesn't deserve that title, often because of very few stupid little things. These stupid little things break things, sometimes directly, sometimes eventually later as the ingredient of some workflow.
To help in fixing this i've written a bunch of e-mails to W3C, OpenClipArt and others, Wiki entries, pieces of PHP script and entries on my SVG links page all for improving SVG validation.
It seems W3C and others can use a hand on this matter, see what you can do, working from this W3C wiki entry on SVG validation