As Erica announced moments ago, the latest beta of iOS 5 (beta 4) is now downloadable from Apple's developer website. There's another way to get it, however, if you're a registered developer with beta 3 installed on your device: over the air.
Beta 4, according to Apple's release notes, is eligible for OTA download on beta 3 devices by choosing Software Update under the General settings pane. Our testers report that it isn't working quite yet, but wait for a bit and try again if you're having troubles.
Up to this point in the iOS 5 development cycle (and in all previous versions of iOS), all updates have been monolithic; developers have to download the full multimegabyte OS to install the new version, rather than a lean delta update with only the changed code. This has caused frustration for those with metered ISP connectivity and others who simply felt it was unreasonable to download a half-gigabyte or more just to patch a PDF security hole.
The preliminary bits hinting at OTA updates showed up a couple of beta cycles ago, but the feature itself is no surprise: it was one of the announced improvements from Apple's iOS 5 presentation at WWDC, although it's not clear whether it will be supported under 3G or only using Wi-Fi connections.
When iOS 5 is released in the fall, it will still require iOS 4 users to download a full version to make the move from 4 to 5; after that, however, the slimmer updates will prevail in most situations short of a device restore.