yes back links are inportant factors for getting a pr ... its one of the inportant factors of ofpage optimazation but there are also many other ways to get a pr or improve a pr

i totally agree with csc gal ... yeh sitemaps are very much inportant for google spiders .. mainly if i say abt xml site map .. its somewhat a techinque webmasters use to show a path to a spider...

u jst visit http://mashable.com/2007/10/20/press-releases/ this url
u will get both pay and free lists of press release sites .. buddy

hey ... its quite right that submitting site to directory for a same theme or niche is quite gaining in terms of pr .. but you should also think about other options like social bookmarking ,...