On this page: http://codex.wordpress.org/Create_A_Network
It says it's not possible to have wordpress installed in it's own directory and create a network because it would interfere with the lookup of blogs directory. However I find this false because if you use subdomains you shouldn't have this interference.
So I gave it a try and it seems to be working.
I had to modify network.php so it would not do that check against the siteurl/home url then when I created the network it said I would have to use subdirectories because of this ... I said fine and continued then
I went to wp-config.php and set SUBDOMAIN_INSTALL to true.
I noticed the main domain was working fine but when I created a subdomain wordpress insisted to set the siteurl the same as it's home url so it was not working. If I tried to set the siteurl to the real url ( domain/wordpressdir ) , wordpress would just replace it with the one without wordpressdir so I went to the database and changed it dirrectly without wordpress and now it's all working well.
Isn't this a bug? WordPress should be able to easily idetify a site by it's subdomain and if that's set up ( SUBDOMAIN_INSTALL ) then it should allow me to install wordpress in a subdirectory.