I don't think this has been posted yet - if it has, my apologies for the duplicate reading I caused...
Got this heads up the other day and am passing it along. Adobe has a special offer right now (Aug 15 thru Sep 15) to allow owners of Elements to upgrade to CS4 for $299 (yes, you can buy Elements now and get the special pricing):
Terms from the site: The US$299 price offer for Adobe Photoshop CS4 standard software is available only to licensed users of Adobe Photoshop Elements for Windows 4.0, 5,0, 6.0, 7.0 or Mac 3.0, 4.0, 6.0 and Adobe Photoshop Elements plus Adobe Premiere Elements bundle who purchase the full version of Photoshop CS4 standard directly from Adobe. The US$299 promotional price is redeemable only from the Adobe.com store or when you call customer service. When purchasing from the Adobe.com store, you must enter directly from your e-mail in order for the discount to be reflected. Offer begins August 15, 2009, and ends at 11:59 p.m. Pacific time on September 15, 2009. Limit one copy of Adobe Photoshop CS4 standard per customer. Offer is not valid to owners of Photoshop Album, Photoshop Limited Edition, or Adobe PhotoDeluxe(R) software. Offer is not valid on Photoshop CS4 Extended. Offer is not valid to education or volume licensing customers, as well as resellers. Prices are in U.S. dollars. All prices are available from the Adobe.com store and by calling 800-585-0774 and are valid in the U.S. and Canada only, excluding Quebec. Applicable local sales taxes and shipping may apply. Allow 2-3 weeks for software delivery. Offer subject to change or withdrawal without notice. Void where prohibited.
From the Adobe web store and many other places CS4 is $699. Plus you could get Lightroom with this deal for $209 (reg 299). So about $509 USD for both.