If you want to know what process has this file open, you can use 'lsof .nfs1234'. Note, however, this will only work on the machine where the processes that has the file open is running. So, if your process is running on one machine (eg. bobac) and you run the lsof on some other burrow machine (eg. silo or prairiedog), you won't see anything.
Here is an example that demonstrates the issue:
Once you have located and killed the process that has the file open, the .nfs file will go away automatically. In the above example, when you kill the tail process, the .nfsC23D file will disappear.