Andrew is having some pain with his I-94, I suspect the first problem is that he flew United who are self admitted cunters. Secondly, you’re meant to hand in the I-94 at boarding not check in (or at least that’s my experience). All is not lost though, apparently you’re meant to mail the I-94 back to them, although that wont be processed in time for your return to the US.
Update: my brother assures me that it’s no big deal to mail the I-94 in, even if you’re returning quickly to the US, as they’re not processed in real time at the airport either. So, perhaps the delay isn’t a problem…