You can now pre-order Mac OS X 10.6 Snow Leopard at Amazon.com for $29. But my advice is to order the Mac OS X 10.6 Snow Leopard Family Pack (5-User) for $49 if you have more than one Mac ($20 more for five licenses? Try that with Windoze). Note that the aggressive upgrade pricing is for Mac OS X 10.5 Leopard users only, and that it is Intel-only (PowerPC not supported). Actual delivery is expected in September.
My advice? Order it, but install it on a non-critical Mac first. At the very least, make a clone of your system before you upgrade, or install it on a separate drive, and let the initial bugs settle out for 3 months or so. And be especially cautious if you use specialized software for things like printers or scanners.