Hello,
I have a very similar problem. I want to use userdirs as in apache and as described here: http://wiki.nginx.org/UserDir and use php with fast-cgi in these userdirs. I've tried it with this:

location ~ ^/~(.+?)(/.*)?$ {
alias /home/$1/public_html$2;
index index.html index.htm;
autoindex on;
location ~ \.php$ {
fastcgi_pass 127.0.0.1:9000;
fastcgi_index index.php;
include fastcgi_params;
}
}

But it get empty pages, when I try to call a php-file.
How can I get php files with match this location to be forwarded to fast-cgi?