Alas, it doesn't have the "print-uris" trick which I relied upon in doing quick upgrades. I've found a simple workaround: use both aptitude and apt-get. The following script updates the list of available packages. It uses apt-get to download -- but not install -- all updates. Lastly, it uses aptitude to upgrade the packages, using the freshly-downloaded files. A "safe" upgrade won't delete a package to let another package be installed correctly, it just does installs.
For a one-line version, see morch's a previous comment.
apt-get -y --print-uris -u upgrade | grep -o -e "http://[^\']+" \
| xargs -r -l3 -P5 wget -nv
aptitude -y safe-upgrade