It would be very useful, IMO, if you provide a "runtime only" installer of Qt for Windows.
Recently there has been a spurt of GPL-ed Qt 4 applications and they are being distributed for Windows also. But each such application ships with its own copy of the Qt DLLs which makes for prohibitive size of each such application installer. If Trolltech provided a neat download for Qt 4 runtimes (i.e. DLLs, translations etc) then this problem could be avoided. All developers would simply ask the users to download and install the Qt 4 runtimes.