In Drupal, theming each node (post) individually was an out-of-the-box feature. Unless I’m missing something, WordPress doesn’t have this functionality yet. Luckly, Justin Tadlock has a perfect solution. You can go through the steps there, but it basically involves including a function in functions.php that checks for a single-post_ID.php file in your theme folder. If you want to completely change the function and form of post #341, just create single-341.php file and theme it as you would a normal single.php file.
For a several examples of why you would want to do this, just visit Jason Santa Maria’s site—it’s brim-full with individually styled posts. Though his website looks to be hand-crafted, it’s easy to do in WordPress now too.