Same speakerphone issues here. Got 2 Thunderbolts last night. Observed terrible echo/screeching on one of them while on speakerphone under multiple conditions - different grips, with/without case, different volume levels, from cellphone or landline as other caller -- all had the problem. The other Thunderbolt was fine. Went into Verizon and they hemmed/hawwed said it's how you are holding phone, etc. Convinced them to do a swap anyway. Second phone was just as bad as the first. Went back to VZW AGAIN, but this time with the "good" phone and "bad" one. They then at least believed there was a problem - the person testing the phone this time spoke louder - I think the caller on the other end's speech volume is also related.
They did one more phone swap and in the test in the store they VZW guy said he heard an echo - I didn't test in the store. At home, however, this 3rd phone seems on initial tests to be much better - possibly as good as the "good" other phone. In the meantime, though, they have filed a trouble ticket - so we will see where that leads.
Not sure it matters, but between 1st and 2nd phones, they moved battery and SD card to the new phone. With 3rd phone, they didn't move anything. Doubtful that such a problem would "follow" a bad battery.