I killed my two biggest annoyances with my G1 with one stone: I recorded a couple seconds of silence and set that as my default ringtone.

• Even if the volume control on the side of the phone gets pushed in my pocket, my phone will vibrate silently (but for the noise of the vibrator) when I receive a phone call.
• The phone cannot by design mix silent and noisy alarms (all can be silent or all can be loud), I can now set the default to noisy, set mid-day alarms to vibrate while using the silent ringtone, and those alarms facing the formidable task of rousing me from a dead slumber can still blast at full volume.

I assume these points are also true for any other Android-based phone on the market.

Detailed instructions

Set silence your default ringtone.

1. Download snd.ogg. You can also record your own silence if you want to be all proprietary about it, but the result should be about the same.
2. Transfer the file from your computer the same way you would any other file. If you downloaded from your phone rather than your computer, I believe it will be automatically added to your music library.
3. Go to the Music application on your phone. In the library, press the “artists” button. There is no artist/album data associated with the file, so Android files it under “Unknown artist” and album “Music”. Scroll down to the song snd. Hold your finger down over it until a context menu pops up. Select “Use as phone ringtone”. Your default ringtone is now silence
4. If you’ve given people in your address book their own ringtones, those will still be audible. Either adjust those settings individually or learn to live with the shame of Hanna Montana announcing your mother’s every phone call.

Silence some alarms.

1. Having followed the directions above, go to the Alarm Clock application. Tap the alarm you’d like to silence.
2. Tap ringtone. In the ensuing menu, select snd. Tap OK.
3. Make sure vibrate is checked to avoid making the alarm too subtle.