Aye. Similar problem here but with 620. After amber update, my phone lost signal easily and oftenly when set to 3G (calls always dropped, no internet connection though showing E). Although, signal was quite constant when set to 2G (no dropped calls, Edge internet connection running).
It's not carrier network related, as when I put the SIM card in wife's 720 (unupdated with amber), 3G signal was fine including internet connection (showing H, H+), no dropped calls.
But then, after 3-4 days 3G signal is back. I did nothing but cursing and several soft restart.
BUT, BUT, BUT, now it gives off a lot of heat when in 3G, and drains the battery very fast. It has been 1 day like that. But this afternoon, the phone seems to be cooler (to be confirmed for the following days).
Hard reset is recommended elsewhere. But I'm too lazy to reinstall apps, maps, etc. So, I will continue cursing and soft resetting, until I'm bored.
Well, that's not helping you, eh? Well, at least you know you're not alone..
If anyone else having similar situation, please share here. With or without solution. At least we know if the problem is limited to a small bunch of people, or in a larger scale.
Amber, amber.. you take my slumber..
Update: Now, the 3G heat and sick-fast battery drain problems seems to be gone. That is after I let the battery phone discharged until it is off on itself (0%). Then charged back until full (100%). Not bad. Although, not so smart update method (amber)..