.st is a top-level domain for São Tomé and Príncipe -- a Portguese speaking nation off the coast of Africa.
Depending on your business and its audience, this may be appropriate. As far as I know, there's no technical reason to avoid using this specific TLD, though.
You can register it here: http://www.nic.st/ Related questions:
Names like del.icio.us are not good because it causes confusion when dictating a url. It is difficult to remember the structure when everyone is used to having a top level domain that is obvious and not phoneticly(SP?) related to the domain. If you are thinking that the domain would be temporary until you can afford the .com, that would likely greatly increase the price as the traffic and value of your site increases.