Let me start this post out with an SEO disclaimer. We can never be absolutely sure that a change we make is the direct cause of change in ranking. Any ranking change might be due to any number of unknown factors, including search engine algo changes, backlinks kicking in that we are unaware of, etc. This month, in particular, Google SERPs have been in such a state of flux, that any changes made to a site during February may or may not be the cause of any changes in ranking you see. Now, having made that disclaimer, let me tell you about a change I made to a site this month that *might* be the cause for a major jump in the rankings.
One site of mine has been doing very well (top 3) in both Yahoo and MSN for quite some time, for its main keyword phrase. However, as is typical these days, it was nowhere to be found in the Google SERPs for that particular phrase. So, earlier this month, I looked carefully at the source code. I divided the code into approximately three equal parts (top, middle, and bottom). I examined the top section, and was satisfied that my keyword placement was satisfactory. I then examined the bottom section, and was also satisfied with it. The middle section, however, contained my keyword phrase but the phrase appeared near the bottom of the section. This particular section had two main elements involved in it, and it was a simple matter of just swapping the elements around. Doing so didn't affect the user experience at all, but did get my keyword phrase to the top of that section of the code. (This may not be an option on all pages - sometimes things cannot be moved around so easily without having negative effects upon what the user sees). I saved the page, and forgot about it. Several days later, my site appeared in the Google SERPs in the number 3 position for that keyword phrase.
Coincidence? Possibly. Maybe it would have ranked whether or not the change was made. Nevertheless, I thought it was worth mentioning, and thought it might be worth your time if you are stumped about what to try next.