Apple has started sending out email invitations to some members of the AppleSeed program, inviting them to beta test iOS 8.3.
Based on the contents of the email — some of us here received it — it looks like Apple is not doing a full-on public beta like they did for OS X Yosemite, but are instead inviting a subset of Apple Seed members to participate in the iOS 8.3 beta. In other words, not every Apple Seed member appears to be getting invited, so you shouldn't join AppleSeed expecting to get an iOS 8.3 invitation.
There's no way to know how many invitations Apple sent out, or whether the company is choosing people at random or has set criteria based on prior feedback from the OS X Yosemite beta. Some of those who did receive the invitation, however, told us they'd been active participants in the past.
So, just to repeat — this is not a public iOS 8.3 beta. This is an invitation-only iOS 8.3 beta being drawn from the public AppleSeed program pool. This is also separate from the iOS Developer Program, which has its own iOS 8.3 beta.
If you've received an invitation, click/tap the "join iOS beta" button to get started. Then please report early and often. For betas, earlier is always better.
Thanks to Keith for the tip!
2pm PT: Updated to explain invitation-only nature of Apple Seed iOS 8.3 beta program.