tedster - 4:13 pm on Sep 4, 2007 (gmt 0) Too small a sample to draw strong conclusions from, but it's worth sharing.
I have two contrasting experiences in this area, from about the same time in 2006. One site had the keyword alt text in place at launch (not present in the logo image), and it never showed a ranking problem. A second site launched with just "Home - company name" as alt text but switched to "Home - company name, keyword text) after six months or so. They're rankings for "keyword text" dropped within a week.
Too small a sample to draw strong conclusions from, but it's worth sharing.