Looks very nice. I hope it will become open source, perhaps I'll even help you with programming it, then.
So, please set it up on Sourceforge or such.
However, as an aside, I wanted to ask you if you knew about mtpaint? I'm a C and Ruby programmer (I know C+++ but I dislike it, it's a beast)
and I wanted to do my own sprite editor for my game too, until I found mtpaint. I looked at mtpaint's source and it's all relatively clean
and portable C, split up nicely in over the c files, using the GTK toolkit. Because mtpaint already has a lot of nice featured, but is also a relatively small
program, I feel that it would be more easy and quick to extend mtPaint to make it more useful as a sprite and tile editor, than to start from scratch.
Basically, the main thing lacking in mtpaint now is are a tile/sprite list like in the bottom of your mockup, and a multi-column display for the palette.