Not strictly true. Although the vast majority of people use alien to
convert from .rpm to .deb - and that's pretty much *all* it's used for
in Ubuntu - it's capable of a lot more.

From the alien manpage:

"alien is a program that converts between Red Hat rpm, Debian deb,
Stampede slp, Slackware tgz, and Solaris pkg file formats. If you want
to use a package from another linux distribution than the one you have
installed on your system, you can use alien to convert it to your
preferred package format and install it. It also supports LSB packages."

The default is to convert the package to .deb format - the parent
package format is automatically detected - but it can convert from/to
any of the listed package formats except LSB, which it can only convert
from, not to.