"Line in the sand" is quite popular recently. Some of my favorites from

[Obamanaut]

the past include, but are not limited to:

'gravitas'

'robust'

and more recently 'just sayin'

[fasttense]

1. That term confuses me.

A line in the sand is easily rubbed out. Why would a line that is easily changed be considered a phrase to explain taking a stand or establishing boundaries. Every time I hear it, I think it is meant as a momentary boundary that will change easily.

From the free dictionary of idioms:
"To create or declare an artificial boundary and imply that crossing it will cause trouble."

"To say that a particular idea or activity will not be supported or accepted."