This doesn't come as a huge surprise given SanDisk's Memory Stick Duo/PRO Duo products have typically been cheaper than Sony's, but they have also announced a 4GB Memory Stick PRO Duo.
According to the press release, this one will be available for a retail price of $239.99 in March. Not only is that cheaper than Sony's offering, it's sooner.
I guess it's also worth pointing out that these cards are in SanDisk's "Ultra II" line, which is geared towards professional photographers. There's a chance SanDisk may also introduce a 4GB Memory Stick PRO Duo in their "normal" line. SanDisk's current price structure indicates their "normal" sticks are, on the average, 25% cheaper than their Ultra II sticks -- if we apply the same pricing system in this situation, a "normal" SanDisk 4GB Memory Stick PRO Duo should go for something like like $179.99.
Competition is good eh, now which would you choose out of the 4GB Memory card, Datels 4gb Hard Drive and the Neo5in1 4GB ?