Hi Simon
By "Absolute timestamps", I'm assuming you mean timestamps recorded by the operating system clock (not the Bpod clock).
With a non real-time operating system, this is a tricky issue. Your best bet, if you need this level of precision, is to use a camera that has TTL signaling capabilities, like Point Grey. In theory, Bpod can manually trigger frames from its 3.3V wire terminals, or ongoing frames can be timestamped by the Bpod clock as Wire1High/Wire1Low events if the camera's strobe output is fed to one of the wire terminal inputs (also see this).
If an IR LED is driven by Bpod, the frame that captures the IR flash should precisely match the Bpod timestamp of the flash. You can actually use this method to measure clock drift, jitter and latency between the two systems, by comparing the OS frame timestamps with the LED flash timestamps.
-Josh