Neat. I like what you did with the mouseovers. There's definitely a lot you can do with simple scripts like this, especially if you craft each post individually.
I was going for something that could be quickly reused without much thought. I'll have to see if my script lives up to that as I continue blogging.
It does have some downsides, and alas, the biggest ones show up in the reblogs of the content on Tumblr — you reblog that post and you see everything. My suggestion would be to see if you could do the collapsible effect with a p title tag, rather than setting two paragraph with two CSS classes, and embedding the Javascript into the post. That way, the effect follows people on Tumblr reblogs.
I'm a bit of a nut for visual effects like this, though. I might try it at some point. Good work.