NEW YORK (Reuters) - As the standoff over raising the government's borrowing limit enters its final month, it's becoming harder for investors to avoid thinking the unthinkable: the world's most trusted borrower could soon renege on its debt. (http://feeds.feedburner.com/~ff/reuters/topNews?d=yIl2AUoC8zA) (http://feeds.feedburner.com/~ff/reuters/topNews?i=S-Yq-ulEOXE:4DRZyD8Okdg:V_sGLiPBpWU) (http://feeds.feedburner.com/~ff/reuters/topNews?i=S-Yq-ulEOXE:4DRZyD8Okdg:-BTjWOF_DHI) (http://feeds.feedburner.com/~r/reuters/topNews/~4/S-Yq-ulEOXE)