To install Swiftdove, the 64 bit optimized version of Thunderbird that includes Lightning, into Ubuntu 7.10 (Gutsy), you need to follow these two steps:
A) Add the swiftweasel repository to apt by adding this line to /etc/apt/sources.list:
deb http://download.tuxfamily.org/swiftweasel gutsy multiverse
B) Update the cache and install Swiftdove:
apt-get update | apt-get install swiftdove-athlon64
Swiftdove is noticably faster, and the inclusion of Lightning is a nice bonus.