Same issue here. Got the pre-release as well as beta. The pre-release gives me the GRUB prompt while the beta just gives me the ISOLINUX line and does nothing after that. BT3 worked fine on this laptop.
The only difference between running BT the last time and now is that laptop has Win7 loaded on it. Thought this should not make any difference as it is booting directly from the DVD.
Any help would be appreciated.