Welcome to the wonderful addiction known as CRACKBERRY, U R amongst friends here so don't ever be afraid to ask questions!
Is it possible your finger is hitting the speaker button as you answer? Been a while since I have ahd a curve but I dont remember seeing a setting where U can set it as default.
I had a simmular issue with my first 8130.
I would answer a call & randomly it would activate the speaker.
Whenever that happened, I knew I had lost controll of my phone & I could no longer hang up or deactivate the speaker. I had to wait for the other party to hang up & as soon as they did, my phone would automatically reboot.
I don't know if this is the same thing happening to your Pearl, but I had to have it replaced & it has never happend since.
I hope a setting adjustment is all you need, but if nothing else works, you might need a replacement.