added note about gcc-4.4 issue to cuda installation instructions... should probably move those...

doc/tutorial/using_gpu.txt

@@ -44,6 +44,18 @@ your computer has multiple gpu devices, you can address them as gpu0, gpu1,
 gpu2, or gpu3.  (If you have more than 4 devices you are very lucky but you'll have to modify theano's
 *configdefaults.py* file and define more gpu devices to choose from.)
 
+.. note::
+    There is a compatibility issue affecting some Ubuntu 9.10 users, and probably anyone using
+    CUDA 2.3 with gcc-4.4.  Symptom: errors about "__sync_fetch_and_add" being undefined.
+    **Solution 1:** make gcc-4.3 the default gcc
+    (http://pascalg.wordpress.com/2010/01/14/cuda-on-ubuntu-9-10linux-mint-helena/)
+    **Solution 2:** make another gcc (e.g. gcc-4.3) the default just for nvcc. 
+    Do this by making a directory (e.g. ``$HOME/.theano/nvcc-bindir``) and
+    installing two symlinks in it: one called gcc pointing to gcc-4.3 (or lower) and one called
+    g++ pointing to g++-4.3 (or lower).  Then add
+    ``compiler_bindir = /path/to/nvcc-bindir`` to the ``[nvcc]`` section of your ``.theanorc``
+    (`libdoc_config`).
+    
 
 Putting it all Together
 -------------------------