tedster - 4:41 pm on Jul 15, 2010 (gmt 0)
There are a number of possibilities - and one major factor would be how critical off-site factors are for that particular query term.
In a one query term economy, there may be a decent number of backlinks for the top ranked URLs, and then on-page changes may not move the needle in any noticeable way. For other query terms, on-page factors could be huge.
For analysis, I would encourage you to think in terms of three buckets, not two, for SEO factors:
on-page (title, headers, alt attributes, text content, and internal linking of various types)
on-site (internal backlinks to the target URL and their anchor text)
off-site (external backlinks and anchor text)
If you are waiting 4 weeks after apparent caching, I'd say that's plenty long enough to assume the changes are incorporated into Google's ranking analysis. One of the areas they talked about with their new Caffeine infrastructure was the ability to make this kind of change faster than it was historically -- updating all of their internal meta-data for any URL almost as soon as they get new crawl information.