If manually typing in
http://mysite.com/photoblog/p/1 gives you a 404 error, than you did not correctly setup mod_rewrite.
Some reason why URL rewrites give 404s
- Either you messed up the .htaccess file
- Your webserver (Apache/IIS) doesn't support mod_rewrite
- Your webserver (Apache/IIS) doesn't support htaccess overrides
And please, provide a real URL, it would be nice to see the error happening in order to solve the issue more quickly and without requiring someone setup the same scenario on there own server.