The term mount in this context is ill-defined. I'm guessing you want to look inside the tarball without extracting it. One handy utility for this is Midnight Commander. See also Wikipedia Midnight Commander page.
This creates a sort of virtual filesystem for tarballs, rpms, deb and all sorts of other archives. Just fire it up, and navigate to your tarball, and hit Enter. To read a file use F3, F5 to copy a file, F10 quit. On Linux at least there is a convenient command help at the bottom of the screen.
So, to summarize you can read the files inside your tarball and copy them from to your regular filesystem. Hopefully that will do you.
I did a little more checking, and it looks like MC is basically only supported on Unix-like systems like Linux, though there is a Windows port here of some sort. However, you don't state what your OS is. I suggest you do so.
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Tar archives are usually not mounted. Did you think of archivemount http://sourceforge.net/apps/mediawiki/fuse/index.php?title=ArchiveFileSystems#archivemount_-_mount_archives_.28tar.2C_cpio.2C_....29 ?
– Benjamin Bannier – 2011-03-14T18:16:14.803