First of all you can download the patched package for Ubuntu 9.04 (i386) and install it. That's it, now your auto-complete is working.
For other Debian-based distros you can patch it yourself:
Install building environment:
$ sudo aptitude install dpatch cdbs fakeroot build-essential
Get sources of QScintilla and step into its source dir:
$ apt-get source libqscintilla2-3
$ cd qscintilla2-2.3.2
Download deb-patch from here and put it to debian/patches directory:
$ cd debian/patches
$ wget -nd http://juffed.googlecode.com/files/03_autocomplete.dpatch
$ echo "03_autocomplete" >> 00list
$ cd ../..
Install all necessary dependencies:
$ sudo aptitude install libqt4-dev python-all-dev sip4 python-sip4 python-sip4-dev python-qt4-dev python-qt4
and start building
$ dpkg-buildpackage -rfakeroot
Now you have you own patched QScintilla built. Install it:
$ sudo dpkg -i ../libqscintilla2-3_2.3.2-0ubuntu2_i386.deb
For those who doesn't use Debian-based distro here is a normal patch here.