They're already in the relative time period when the PS1 had gone to $149 (~3 years in North America), and getting towards when it went to $129, so the PS2 has been riding "higher than it should" for a bit already. (Though admittedly having other competition at that level keeps it up as well.)
In all likelihood they want to see how a few months measure out before making the decision--a price lowering always causes a surge, and it takes a bit to see where general sales trends are sitting at after that point. (Just as I'm sure Microsoft waited out the GameCube's $99 surge to see where they stood in comparison after that, which in the end did prompt their price drop.)
We'd been expecting summer price drops for the Xbox and PS2 for a while, so it certainly wouldn't be out of line. (And I would certainly rather drop in connection with a major gaming event like E3 if doing it at that point, since you get more hub-bub.) Perhaps Sony will decide whether or not E3 is the best time to do it, even if it's a bit earlier than they want, simply to take advantage of the event. (And if they were planning on going to $129 at the next drop anyway, it would cause a good commotion from their leapfrogging.) So the early Xbox drop might cause them to switch earlier than they'd thought to before... but they may well ride it out another few months to make for a late-summer event, as by that point they would be able to see the real effect of their price on sales, and hit the exact number they want.
Since it's also good to be seen as "leading" rather than "being forced" to do one thing or another, they may well hold off a bit for that reason as well.
I do expect them to drop at some point soon, though, and have been expecting that for about the past year. Heh...