...I forgot to add that I've tried importing the project in both versions and get the same result.
A project that you created in After Effects CS4 is not opening in either After Effects CS4 or After Effects CS5?
If so, please send the project with a bug report to aebugs -at- adobe -dot- com.
Will, I've solved my immediate problem at least. Luckily I "saved as" a different name at one point and it opened fine. ...not sure what's up with the original file. Anyway, thanks for the help.