It is out. Look for OCZ RIMM4800. It's on their website and it's on pricewatch... just no one seems to care anymore.
As for 512 not being enough, well, that's what I mean, it's about enough for today's apps but some people like more, and many on this forum have reported that the latest games such as Battlefield 1942 will suck up 512 and improve performance with 768 or 1024. Plus, for memory-heavy applications such as Photoshop or DV editing, 1024 is considered the standard.
512 is about enough for today's applications but with falling DDR prices many are going to have more soon and the performance difference is going to start showing itself. At least IMO, 32-bit RDRAM would do well to become as cheap in 512 denominations as it is to buy 2 sticks of 256.