Is it possible on a Droid X to repeat the notification sounds on a regular basis (perhaps every 5 minutes or so)? When I get a voicemail, I want it to beep every few minutes to remind me to check it.
Missed Call will do repeat vibration patterns/notifications for SMS (per contact), missed/incoming phone calls (per contact), Battery, Calendar events, device storage, etc.
Unfortunately this doesn't do voicemails, but if a missed call is good enough for you, then give it a shot. You can also set it up to do different patterns/notifications per contact for missed calls (so alert you differently for a VIP than everyone else).