Create on insert a unique random number

<div class="Section1"><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">When inserting a record is there a way to have postgres create a random number for a field such that
itis unique?</span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt; 
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">Thanks, </span></font><p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">Lance Campbell</span></font><p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">Project Manager/Software Architect</span></font><p class="MsoNormal"><font face="Arial"
size="2"><spanstyle="font-size:10.0pt; 
font-family:Arial">Web Services at Public Affairs</span></font><p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;font-family:Arial">University</span></font><fontface="Arial" size="2"><span
style="font-size:10.0pt;font-family:Arial">of Illinois</span></font><p class="MsoNormal"><font face="Arial"
size="2"><spanstyle="font-size:10.0pt; 
font-family:Arial">217.333.0382</span></font><p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">http://webservices.uiuc.edu</span></font><p class="MsoNormal"><font face="Times New Roman"
size="3"><spanstyle="font-size: 
12.0pt"> </span></font></div>