Maemo is based on Debian and uses the apt packager manager to fetch *.deb files from a central repository. The install files only contain the URL to the repository and the package name. The packages for everything you've installed is kept in /var/cache/apt/archives on your device and you can backup that directory in case the repository ever gets shutdown and it will probably be shutdown in the next couple years.

If you ever need to reinstall those files from your backup just run "dpkg -i *.deb". That will install every one of them which might not be what you want since packages you uninstalled and no longer want might be in there. The other option is to give it the exact filenames of each package or delete any packages you don't want. Some of the packages depend on a lot of other packages that you didn't directly install, so keep deleted packages around in case a package fails to install.