Lion introduced a new Time Machine feature the OS would automatically create local snapshots of files so that even without a backup disk attached, you could still enter time machine and recover or undo changes to certain files. As your disk space becomes low, this space is automatically freed and the rate of snapshot creation is reduced.
This is great on desktops and I would assume the newer Macbook Pros that have better disks, but on my older MacBook Pro I've noticed that Safari has had trouble loading web pages and during this time, I hear the disk thrashing for about a minute or so and then all of a sudden the page loads. After a bit of researching and some help from the 'fs_usage' command, I discovered that a process called 'mtmd' was accessing the filesystem often and it turns out that mtmd is responsible for creating these local time machine snapshots. A quick Google revealed how to disable the local snapshot functionality:
sudo tmutil disablelocal
We'll see if that helps the disk thrashing! I did immediately recover 1GB of disk space though which is a good sign.