Referring to /usr/share/doc/base-files/README:
Q. I upgraded my system to the unstable distribution and now my /etc/issue says "wheezy/sid". Should it not read "sid" or "unstable"?
A. You obviously do not understand how the testing distribution works. Packages uploaded for unstable reach testing after ten days, provided they are built for every released architecture, have no RC-bugs and their dependencies may be met in testing. You should consider the testing and unstable distributions as two sides of the same coin. Since the base-files package in testing was initially uploaded for unstable, the only sensible /etc/issue to have is one that is both valid for testing and unstable, hence "wheezy/sid" (or whatever is appropriate).