Looks like InfinityWard is working hard on getting their bugs fixed for Call of Duty 4 as they just released patch 1.2.

This patch optimizes the Lean view offset issue, addressing the glitch reported by several community members and documented and sent in directly by community member GaretJax which is commonly referred to as the “Left / Vertical Lean” Glitch, in addition to preparing files for compatibility with the future release of the Call of Duty 4: Modern Warfare Mod Tools. This patch will update all previous versions of Call of Duty 4: Modern Warfare to v1.2