When I incorporated Jabber our setup I had a whole lot of already registered users that I needed to register with my Jabber server and to do that I ran a script that would do just that.
The second record in the registration script threw this error
Ejabberd Resource Constraint: Users are not allowed to register accounts so fast
The error occurs because the default configuration allows only 1 user in 10 mins to register with the ejabberd server from a given ip. If all your users would be registering from your server then this would surely become an issue. The solution for this is to add this option to your ejabberd.cfg file
This simply eliminates the time limit.
This might come in handy too http://www.ejabberd.im/node/3003
Though this issue is documented on the ejabberd.im site I am just adding this purely for my own reference and for others who stumble upon this as the first resource.
Hope this helps.