May Security Update Released for WP7 [Updated--up to the carriers]
Looks like the planned May update is being rolled out, version 7.0.7392.0. This update addresses the fraudulent third-party digital certificates that could lead to potential phishing attacks or intercept web browser connections (man-in-the-middle). The update simply moves these certificates into the "Untrusted Publishers" certificate storage on your device.
More information on the update is available over on the update history page. Have you received the update notification yet?
Updated: According to Microsoft, this update is at the discretion of carriers, again, meaning some of us may not see this until it is tied with another, future update: "How you get 7392 depends on your mobile operator and what updates you’ve installed...If you’ve already installed the March update, you’ll receive 7392 as a standalone download or bundled with a future update."