We just passed the 50,000 photo mark.
Here is what I wrote back on the 24th November 2008:
That makes it 226 days to upload 30,000 photos
The 50,000th photo uploaded was probably this one from Calgary Zoo, uploaded by Ituri (see note below):

(note that due to deletions and non-zoo related photos in the gallery, this is not 100% accurate)
Here is what I wrote back on the 24th November 2008:
Just thought you might be interested to know that we have just passed the 20,000 photo mark in the gallery
It took:
766 days to upload our first 5,000 photos
107 days to upload our second 5,000 photos
42 days to upload our third 5,000 photos
79 days to upload our fourth 5,000 photos ... we slowed down a little - but then I also had the site offline for a while and the gallery was also down for some additional time.
Well done everyone.
That makes it 226 days to upload 30,000 photos
The 50,000th photo uploaded was probably this one from Calgary Zoo, uploaded by Ituri (see note below):

(note that due to deletions and non-zoo related photos in the gallery, this is not 100% accurate)