It came up on Wikipedia:Talk:Gaussian blur that The GIMP cannot reproduce the look of a an out-of-focus image because this is not the same as a simple gaussian blur. At first I thought it was just a circular box blur, but that also fell short. I was able to reproduce the look nicely as follows:

I took an image in GIMP, set the gamma very high, saved that, opened it in Image J, convolved it with a 41-pixel–diameter circular kernel (which took a while since it's a O(x·y·r2) operation), saved that result, opened it in the GIMP, reversed the gamma transformation (which was required to get just the bright spots to bloom into circles), then finally I overlayed the unblured version and masked the portion I wanted out of focus. I tweaked the results (adding noise, and a tiny bit of gaussian blur), but what I describe got me 90% of the way from the original to the final version. The pictures can be found here: http://en.wikipedia.org/wiki/Talk:Gaussian_blur#Out-of-focus_lens 

The python script I used to make the convolution kernel goes like this:

#!/usr/bin/python
import math

size = 41
mid = size/2
for x in range(0,size):
  for y in range(0, size):
    dist = math.sqrt(pow(x-mid,2)+pow(y-mid,2))
    if dist < mid-0.5:
      print 1,
    elif dist > mid+0.5:
      print 0,
    else:
      print dist-(mid-0.5),
  print ""


This might better done with Python-Fu, but I'll start with just a bug report.