I've been having the same problem too. As I sit here typing it is switching from full 3g to no signal to 1x randomly. I reckon its a vzw issue.
I see the same issue from time to time. Yesterday it went into 1x, after 30 minutes it still wouldn't revert back to 3g, so I put it in airplane mode, then turned AM off and it fixed itself.
mine won't even say 1x, the 3g/1x icon disappear completely and the only thing i can do at that point is text and phone calls. i switched to BB 1.1 with the chevy 1.2 kernel last night and the problem still persists. 3g has been up for a couple of hours now, but i did d/l the t3hsteve and adamz kernels just in case to give them a try if it continues to happen today. i guess i'll also try giving verizon a call and see if it's a known issue with them.
The way that Verizon authenticates 3G is different than the way it authenticates 1x. In order to connect to the 3G network, they use what is called a DMU key, which is basically a small file that resides on the radio portion of the phone that contains an authentication string.
I first learned about this with my old Palm Treo 700p after I completely erased the nonvolatile RAM in the CDMA radio and subsequently lost the DMU key. I was able to connect via 1x, but not 3G. Eventually using a special serial cable that I built myself, as well as some programming software I found on the internet, I was able to upload the DMU key back to the phone, thus restoring the 3G connection.
Now i'm not sure where this file is located on the Droid - on the Treo is was in a section of RAM that was completely independent of the PDA portion of the phone, so it was never affected by any of the system files (And custom ROMs were impossible for the Treo 700p anyway).
This may not even have anything to do with what is causing the Droid to randomly drop the 3G connection, but knowing that verizon uses two different authentication methods for 3G and 1x might shed some light on the issue.