Thanks for the great comments!
I made the background using the following simple process:
1. I find a nice texture that I like. I find the stuff at http://cgtextures.com can be good, but there are other sources for nice big free paper textures if you look hard enough. This seems to be potentially a useful post too: http://slodive.com/freebies/158-paper-t
s-designs/
2. I opened the paper jpeg up in Gimp and convert it to greyscale. Even maybe invert it. Play with it a bit using levels etc. Depends. But I want to end up with a greyscale texture of some sort.
3. I bring that greyscale texture into Inkscape and use it as a mask over my plain coloured rectangle (Object->Mask->Set). Depending on how you do step 2. you can get different effects. The darkness of the mask image decides how opaque the colours in your inkscape object show up. It's good for mimicing textures.
I used the same exact technique (with a much less subtle texture) for these ones a while back: http://crunchbanglinux.org/forums/topic
t-colours/
It doesn't always work great (that step 2 makes a huge difference to the outcome).