Unique random string

Any ideas on how I can generate a 'guaranteed' unique ascii string less than 15 characters. It must be guaranteed unique across several independent servers. Currently we are generating random short strings to create unique aliases for email addresses. The only problem is that to guarantee uniqueness we keep a log of previous strings and check to make sure we don't create duplicates (we cannot rotate strings. They must be absolutely unqiue for the next decade or so). The optimal solution would be to generate strings that are truly unique and hopefully random enough, w/o having to go through the process of checking for duplicates. I'd use uniqid(), except that it is 32 characters (too long since users will have to handle them sometimes) and the algorithm must be available to C, C++, Perl, whatever. Any comments, suggestions or answers! ;) would be great. Please add my address to the reply as I am not subscribed to the list. TIA, Bill