Interesting effect. I've done things similar to that in the past on my Tumblr. http://sfbne.ws/PH8kO8

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.