Every time I re-open Unity, I have to rebuild one of my external DLLs

I have a DLL that I'm including in my Unity project. Every time I re-open Unity, though, I get the following error:

FileNotFoundException: Could not load file or assembly 'DataCommon, Version=1.0.5012.34874, Culture=neutral, PublicKeyToken=null' or one of its dependencies.

DataCommon.dll is exactly where Unity thinks it is ("Show in Explorer" even works properly), I can see it right now. If I rebuild DataCommon and overwrite the DLL, the error will go away and everything will work properly, but if I then close Unity and re-open it, I'll get the error again.

What might cause behavior like this? I'm not really sure how to address this. Nothing looks out of place, and as I said if I just rebuild the DLL (copying over the same copy doesn't work, it's like it needs to increment the file's updated time), there are no problems.

I've tried updating to the latest version of Unity Pro (4.2.1f4, previously 4.2.0), but that didn't help. I'm running Windows 7, 64-bit, and the DLL in question is targeting .net 3.5.

I appreciate any help or suggestions.