God's faithfulness to us is independent of ours to Him. He can't be unfaithful. It's just not in Him.
My wife's coming home today from Haiti. She's coming home with scabies or some fungus on her skin, not unlike ancient lepers. But she's also bringing home a sense of God's faithfulness in sustaining her, hearing her, providing for her, using her, and on an on.
The amazing thought in that is that God does this all the time. He's 100% faithful, not 99.9999%. He never fails. He never forgets. He never falters. He is always faithful. And this faithfulness is an attribute of Him - it's in His character. To be unfaithful would be to deny Himself. Whether or not we're faithful, He is. Whether or not we stick to it, He does. Whether or not we cling to Him, He's faithful. Whether we fail and fall to temptation or walk with clean hands and pure hearts before Him, He's faithful.
If we are faithless, He remains faithful - for He cannot deny Himself. (2 Tim. 2.13)