Are you using a recent version are of R?
With R 2.7.2 I was able to load a csv file with column names longer than 256 characters (tested with up to 312). However, showing the object (e.g. dat <enter>) did give the following error message:
Error in do.call("data.frame", rval) : symbol print-name too long
although str(dat) works ok.
Do you actually have long column names in the csv file, if so you could try shortening them to less than 256 characters. If your column names are not long it may be that the csv file is not using comma separators. It would be helpful to have some more information about the structure or your csv file and if possible post a small example file that gives the same error.