7.1 Beta 1 & table size calculation

I am looking at 7.1 beta 1 to see how my application (currently on 7.0.3) will operate under 7.1. To calculate used space, in 7.0.3, I lookup the relpages count for the table in pg_class and multiply it by 8 (8k is my block size). In my testing of 7.1 beta 1, I created a record of length 24947, but the relpages count is only 1 (after a vacuum analyze, and with block size of 8k). Is the relpages counts under 7.1 no longer valid of the physical page storage??? and what would the correct way be to calculate this under 7.1 beta 1 ????

Thanks in Advance,
Dale.