I haven't seen either of these things before and I did some searching and still can't find anything. You can search the forum for 'Adsense' and find some threads about placing ads. The first request sounds like a nice feature. It would probably be a very involved hack/plugin, though.

you can add content between posts (not comments) by altering the posting date on teh new post you wish to put in between the other two

so say you just launched your blog on 7/22

Post 1 = 7/22
Post 2 = 7/24

but on 7/25 you want to stick a post inbetween posts 1 and two, then just edit the date of posting to 7/23 and be sure to check the box next to it, to edit the time stamp, and you're done. hope this helps.

I think jamigre means adding code for something like an advertisement and the code always shows up between the second and third post, not just another post. The problem is that we can't just open up _main.php and drop in the code we want because it would have to go in the loop that displays blog posts. If we put the code there, then it would show up after every single post. Here's one option in pseudocode:

while ($Item = $Mainlist->get_item() ) { // the loop begins

// all the code for displaying a post goes here

// just before the loop ends and starts over with the next post we put in a clever bit something like this

if (!isset($after_post) ) {$after_post = 1;}  // set it to one the first time
$adfile = 'ads/afterpost' . $after_post . '.php'; 
include($adfile);
$after_post++; // increase by one for the next time

// then the loop ends
} ?>

Then you would create a folder in your skin's folder called 'ads'. Inside you would create 'afterpost1.php and put any code in there that you wanted to show after the first post. 'afterpost2.php' would be for after the second post. You could just create the files you needed (or create empty files if you're getting not found errors.)

I'm not sure if this will work. I haven't tested it. If you think it might be worth a try, then back your stuff up and try it. If you're still interested, then I'll help debug the code. We could probably write a pretty front end for managing your afterpostx.php files. It could show up in the backoffice if you don't want to be opening files and editing code.