I am part of a small group beta testing software that allows modification of the native Kindle 2 fonts. It actually works. Please see the following page for examples.
In this case, I switched the serif fonts for a sans-serif font to get around the "whispy" font issue. I also changed the native sans-serif font for a darker one.
This is but one example. An updated font could be any TrueType font.
During the testing, I found that Amazon's default font is superb. compared to any other serif font, at the sizes used for the K2 screen, the Kindle's native font is vastly better. So, while we may complain about it, it really is an extremely good font.
Here's the link to the samples:
Edited: I should not that these are by no means optimized. I just picked some fonts that had the basic characteristics we were looking for. However, I think the bolded sans-serif font is too bold. I'll be on the lookout for some good fonts and will post more pictures as the experiments progress.