Well, according to Jewish tradition there's at least one occasion in the Bible where God does lie. And it shows that lying isn't always bad.
The scene: Abraham has just invited in three strangers and fed them, when one tells him his wife, Sarah (who is 90 at the time), will have a child. Sarah is inside the tent, and overhears the conversation:
Gen 18:12"So Sarah laughed to herself as she thought, "After I am worn out and my master is old, will I now have this pleasure?"
However, when God speaks to Abraham, what does he say?
Gen 18:13 "Then God said to Abraham, "Why did Sarah laugh and say, 'Will I really have a child, now that I am old?'"
Notice what's been done here. God has misreported Sarah's speech in the tent, where she claimed Abraham was too old to have children (he was 100 years old), instead leading Abraham to believe that she said she was too old.
In Jewish Biblical analysis, precise wording is important. So when God applies the verb "has become old" to Sarah while she actually applied it to Abraham, it's understood to be a substantial change in the meaning.
This is a text commonly referred to in Jewish ethics, and teaches an important lesson about delicacy; that it's sometimes OK to lie if it stops someone getting needlessly hurt, and that sometimes there are higher values than truth. One such value is peace, such as the peace between a husband and wife.
Rather than upset Abraham about what his wife said, and potentially cause an argument, God chose to.. erm... bend the truth. It's something we all do every day when asked 'Do I look nice?', or 'Did he say anything about me?', and it's morally the right thing to do. Some ethical codes like Immanuel Kant's Categorical Imperatives would say that one should be truthful in these cases. Not this one.
Of couse, this shouldn't be taken as a liar's charter. Truth is still seen as an important Jewish value. The Rabbis severely restrict when lying is acceptable, and disapprove of it even then. One may lie to bring peace between arguing people, to protect someone's privacy, or to be humble. But if one gets into the habit of lying, even for these reasons, it's seen as a negative thing.
So God can lie, but only white lies and only for the right reasons - and this doesn't undermine omnibenevolence.
www.yeshiva.org.il/ask/eng/print.asp?id=1003