If you like to hack, have a look at mortscript
http://www.sto-helit.de/modules/edito/content.php?id=38
Here's two simple one liners I just whipped up
messageOff.mscr:
RegWriteDWord HKCU,\ControlPanel\Notifications\{A877D65A-239C-47a7-9304-0D347F58
0408},Options,8
messageOn.mscr:
RegWriteDWord HKCU,\ControlPanel\Notifications\{A877D65A-239C-47a7-9304-0D347F58
0408},Options,9
Put these in your Launcher folder (if you're using Seligman's Launcher) named 00messageOff.mscr and 01messageOn.mscr
They'll show up as the first two launcher items in the Today launcher icon view...
Tap the first one to turn off notification sounds for new mail messages, the second to turn it back on. You should check with a registery editor if the keys are the same, I only have my Sprint ppc6700 to test with it, but it works on mine.
Why do this? I like to have a little beep play when I get new mail, but not so much at night while sleeping
so I have to turn it off every night, and back on in the monring - icky. These are quick one tap on/off commands!
You could add whatever other notification keys you want to toggle on/off to this script, they're all in that HKCU,\ControlPanel\Notifications registry
the optons value seems to be a bit field
8 means 'display message' is checked
1 means 'Play sound' is check
so, 9 = message+sound, 8=message only.
It might be possible to wite a script that runs forever, and just turns notification sound off at 9pm and on at 8am (or whatever) but for now, baby steps