== The ending ==
In case it doesn’t all fit together…
The Grebulons were set up by The Guide MKII to blow up the Earth after all of the stray earthlings were back on it. The astrology wasn’t important, but it may have helped them to decide to blow up the Earth. The Grebulons were set up by the Vogons who took over the Guide and made The Guide MkII?. The psychologists hired the Vogons to destroy the Earth because knowing The Question of Life, the Universe, and Everything would put them out of a job. The Earth was made to find out The Question of Life, the Universe, and Everything. The first time the Vogons blew up the Earth, they failed to do it throughout the Whole Sort Of General Mish Mash, and Arthur and Trillian utterly failed to be blown up at all. This was supposedly fixed at the end by The Guide MK II (which made the destruction of the Earth inevitable in all realities), and so the Earth was finally destroyed, the Prostetnic Vogon Geltz could check off the little box, and the psychiatrists could live knowing they were going to have jobs for as long as they cared to live.
Now for Arthur. After meeting Agrajag, Arthur knew he couldn’t die until he had been to Stavromula Beta. At the end, they all met coincidentally (as a result of The Guide MkII? as described above) and Arthur ducked when he was shot at. The guy who got hit was, of course, Agrajag, and commented on it being Arthur who killed him again. Then Arthur picked up a card from a table and noticed what it said. Stavoro Muller: Beta. Notice the similarity to Stavromula Beta. There was now nothing keeping Arthur from dying, so he figured that there was no way out of this one.
This answer was thoroughly nicked from Brian Kofford, so if you have a different take on what happens, make add it, or make a post in the newsfroup about it.