1.4RC3 is in the scribus own repositories...
http://wiki.scribus.net/canvas/Ubuntu
1.4 will be there, too. as soon as it is released.
I am one of the chief participants in a local "Linux Clinic," held on Sunday afternoons. Yesterday we had a gentleman who wanted to do a newsletter on a computer that had Jaunty 9.04. He was trying to do it in OOo, so I turned him onto Scribus.
I helped him add the Scribus repository (following instructions on the wiki), but after reloading all that showed up was 1.3.5 (as well as 1.3.3.14). I had him go ahead and install 1.3.5 just so I could give him a quick tutorial to get him started. I also strongly recommended that he do a couple dist-upgrades to Lucid later, as we didn't have time at the Clinic.
What I don't understand is that, after adding the Scribus repository, why didn't 1.4.0 RC3 show up?