I just downloaded the installer today, and Avast! warns me that it contains a virus/worm named "Win32:Trojan-gen. {other}"

You may want to investigate immediately!

It's a known false alreat created by AVAST.
Please contact them for the update.
AND ASK TO PUT THIS INFO INTO THEIR SITE, PLEASE.

Specifically, the warning popped up on the following filenames:

6/18/2007 9:53:30 AM SYSTEM 1536 Sign of "Win32:Trojan-gen. {Other}" has been found in "C:\...\NuSphere-phped-5.0-php4.4.6-Win32.exe" file.
6/18/2007 9:55:57 AM SYSTEM 1536 Sign of "Win32:Trojan-gen. {Other}" has been found in "C:\...\NuSphere-phped-5.0-php5.2.1-Win32.exe" file.
6/18/2007 9:56:04 AM SYSTEM 1536 Sign of "Win32:Trojan-gen. {Other}" has been found in "C:\...\Mozilla-browser-2.0-Win32.exe" file.

OK. Thanks.

In the meantime, is there anything you can do that would produce a package without the warning? It might prevent people like me opening more tickets.

no.
Avast triggers on the Inno Installer code which can't be changed.
and please do not proceed until you get the update from them. It's quite important to be up-to-date

This was just resolved by Avast! with an update to their detection library. (Detection library 000750-1 or newer)

You should now be able to install without virus warnings.