I’m not sure what to advise without having the exact text you are trying to post. In its original file, just to be sure that no funny characters are lost. Yes, even Plain Text files can have hidden characters in them.
Otherwise, it is just wild guesses.
I’d then be trying to recreate your problem on a clean install of WordPress 3.0.1 with no plug-ins and the default theme. If it fails for me, I’d start by looking for invisible characters using an editor like NotePad++ with the display turned on for special characters, end of line, etc., and be looking right around the point the post stopped. I’d also be looking at the possibility of the whole post being one long “line” without any line breaks.
If it didn’t fail for me, then I’d get you to create a clean install of WordPress on the same web site (in another folder), and test it there. If it failed, then there is something about your host that is different from mine, and you’d probably have to talk to your host about that. If it worked, then it is a matter of determining what theme or plug-in is at fault.
At the end of some of these paths comes reporting the problem as a WordPress Bug.
Of course, I may have missed some possibilities.
If you are up to it, you are, of course, welcome to explore any of these paths yourself. You could, for example, setup an account on wordpress.com and try your post there.