ubuntu 11.04 has native python2.7 i build python2.5 from source to /usr/local/python2.5/bin, and try to install lxml for my custom python2.5 install. Also i use virtualenv.
try this to install lxml under your virtualenv, hopefully you won't get any errors
/home/se7en/.virtualenvs/e-py25/bin/activate
easy_install pip # if you don't have it already
pip install lxml
python -c 'import lxml' # to confirm all is good