If speed or memory is an issue, one could avoid loading the entire file into memory as follows:

my $last_id;
my $last_date;
my @times;

for (;;) {
   my $line = <DATA>;

   my ($id, $date, $time) = split(' ', $line||'');

   if ($. != 1) {
      if ((!defined($id) && @times)
      ||  $id   ne $last_id
      ||  $date ne $last_date
      ) {
         print(join("\t", $last_id, $last_date, join(',', @times)), "\
+n");
         $#times = -1;
      }
   }

   last unless defined($line);

   $last_id   = $id;
   $last_date = $date;
   push(@times, $time)
      if !@times || $times[-1] ne $time;
}

__DATA__
848    05/23/06    11:00
848    05/23/06    12:30
848    05/23/06    13:00
848    05/23/06    14:00
848    05/25/06    11:00
848    05/25/06    12:00
261    05/24/06    11:00
261    05/24/06    12:30
261    05/24/06    13:00
261    05/24/06    13:00
261    05/24/06    13:00
261    05/24/06    13:00
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It also preserves the order of the ids in the output.