It is a namespace thing. I am pretty sure. When I use a title for my posterous posts that no one ever used before, the permalink is just the title. When I for example post something titled generally like "photo" it becomes "photo-73864"
if you check for example: http://posterous.com/explore
You can see all pretty unique titles without a number appended and then a post titled "Tetris" gets "/tetris-194"
Even if you post something without a title it just gets an ID assigned which is (this is a guess tho) the unique ID in their database of all blogs.
