Registering 2 letter second level domain

Hi!

Does anyone know if there is any imperative (writen or oral)
to avoid registering 2 letter second level domains? 

I know some top level domains refuse to do so based on possible
conflicts (due to user misconfigurations) with current or 
future top level domains, but others allow it.

We have a request to register one of these under "es" and
I want some advice before saying "yes" or "no".

Thanks,

Miguel


+---------------------------------------------------------------+
|   Miguel A. Sanz              Tel:    + 34 1 5855152          |
|   RedIRIS/CSIC                Fax:    + 34 1 5855146          |
|   Serrano 142                 Email:  miguel.sanz at rediris.es  |
|   28006  Madrid                                               |
+---------------------------------------------------------------+