I'd like to buffer input from a serial device (/dev/sda1). I could alter my program, but I'm wondering if I can create a special file/device which will buffer that input for me?

Serial device is like /dev/ttyS0.
/dev/sda1 is scsi hard drive #1, partition #1.
The serial input is buffered in the kernel. Not sure about the size but there is something like 2-4 k bytes buffering.

Whoops; yeah I meant /dev/ttyS0

Are you sure it's buffered? I mean, the way I think of it, I'd want it to wait to give me the data if I'm not currently reading from the device. But it seems like if I'm not reading from the device when the data arrives, I don't ever get it. Maybe that's baloney? Maybe I should test it out again?

Its buffered for sure, up to some limit. Most serial libraries flush the serial buffer when they open the port. The kernel ISR places all RX data in the buffer, and read() pulls the data from the buffer, read() doesn't read from the port directly. If the port is open, and you don't close it, then you should get the data that arrives since the last time you tried called read().
You can look at my serial library here,

Edit: my website/email is firewalled and only available to Cont. US. If you're outside post a message here and I'll move the files somewhere else.