Davis DVD usually get the earliest (and most reliable) DVD news, and they say that 300 is tentatively scheduled to come out on DVD, HD-DVD, and Blu-ray disc in August. The release window should be roughly the same as V for Vendetta when the DVD is officially announced.
And considering 300 was a box-office hit, I wouldn't put it past the WB to offer up single-disc and 2-disc editions of 300 on standard definition (I would be very surprised if they only offered a 2-disc edition), a DVD/HD-DVD combo disc, and a regular Blu-ray release.