More or less, yes. I setup my Extreme last night with a 320Gb Iomega MiniMax that was also received in the same order. When the system (with drive plugged in) was started up after I had loaded the drivers and utilities for the Airport Extreme the disk was connected with the full 300Gb capacity. However, I discovered that I wasn't able to partition the drive over the network so I attached it directly to the MacBook Pro via FireWire and set 2 partitions on it using Disk Utility (first time I've used it for partitioning). Anyway, when the disk was reconnected to the router it didn't recognise the hard drive connected to it and this caused all sorts of issues over the next hour. In the end, however, turning the Airport Extreme off (a software restart doesn't work) did finally resolve the issue so now I see 2 network locations when I connect to the Airport Extreme.
The moral of this story: turn off the Airport Extreme before connecting a disk drive to it.