Announced exclusivly at the All About Symbian Pub Meet this evening is Chris Davies' minimalist but useful Web 2.0 utility, Why The Frak Won't My Sis File Install?  Although Rafe would like to point out that if you work for Symbian it might be NSFW...

I tmight seem repetitve, but the site allows you to uplaod a SIS file you are having trouble with (built for 9.1 or later) and gives a diagnostic on the problem. Hopefully it should help your debugging become a little less stressfull.