I've read snippets around here that seem to suggest that at this time you can only connect one BTLE (BT4, BT Smart) device to the iPhone at any one time. As the Pebble uses a BTLE connection to the iPhone to make notifications work, this would potentially stop a BTLE heart rate monitor from connecting to the phone.
I haven't found anything about this limit on the BT website. However, BT4.1 has been announced and should be provided as an over the air update for existing BT4 chips. BT4.1 comes with added functionality that should allow various BTLE devices to coexist.
But then again, this "limit" may be complete cobblers and it may be a completely different issue.