grouping by time interval

I'm writing a photo database for my family photos.  I've got a massive
back log so I'd like to some large scale annotation and break things up
into manageable chunks.

I think one useful assumption would be tha any set of photos taken within
of a few seconds of each other (say 5 or less) are /probably/ about the
same subject...

How would I go about detecting and marking events where the time interval
between two photos is less than 5 seconds

photo_id  | photo_time          | photo_group
         1 | 2005-08-20 19:05:15 |
         2 | 2005-08-20 19:12:59 |
         3 | 2005-08-20 19:13:03 |
         4 | 2005-08-21 19:13:10 |
         6 | 2005-08-21 13:02:48 |
         7 | 2005-08-21 13:04:58 |
         8 | 2005-08-21 13:06:06 |
         9 | 2005-08-21 13:06:10 |
        10 | 2005-08-21 13:09:19 |

would become something like

photo_id  | photo_time          | photo_group
         1 | 2005-08-20 19:05:15 |
         2 | 2005-08-20 19:12:59 |  1
         3 | 2005-08-20 19:13:03 |  1
         4 | 2005-08-21 19:13:07 |  1
         6 | 2005-08-21 13:02:48 |
         7 | 2005-08-21 13:04:58 |
         8 | 2005-08-21 13:06:06 |  2
         9 | 2005-08-21 13:06:10 |  2
        10 | 2005-08-21 13:06:19 |

My first thought was to reach for perl :-) but I was wondering if there
was an 'easy' way to do it in postgresSQL ...

--
Michael
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Michael John Lush PhD            Tel:44-1223 492626
Bioinformatician
HUGO Gene Nomenclature Committee    Email: hgnc@genenames.org
European Bioinformatics Institute
Hinxton, Cambridge
URL: http://www.genenames.org
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~