I was confused was like almarma. I was on 3.1 and wanted to try 3.2 beta via automatic updating with the Beta Tester plugin. With Point release nightlies selected it linked to "3.1-latest.zip". Which didn't seem to be what i wanted. With Bleeding edge nightlies selected it linked to "nightly-builds/wordpress-latest.zip". But that didn't seem right either. I didn't want nightlies — i wanted the beta "point release" aka 3.2 beta 2.
Eventually i just tried them both. First the Point release nightlies, which put me on 3.1.2 something — which is older than the current stable 3.1.3 (latest.zip). Odd.
Then i tried the Bleeding edge nightlies option. It installed ok, but at the bottom is said i was on 3.1.2-alpha.
It wasn't until i clicked to the main dashboard page that it said i was on 3.2-beta2-18145. And the "3.1.3 update available" notification was still displayed on a couple pages.
It all seems far too confusing. I get that the plugin name probably implies "beta testers" in the generic sense, not necessarily a specific "beta release". And perhaps the nightlies aren't any more scary than the labeled beta downloads. But it can be misleading. I don't always want to run nightlies, but i am more comfortable installing official betas and release candidates. Perhaps a new update channel could be added to the plugin for pre-release point releases like betas and RCs.