There are many factors that can come into play here. The 10.6.8 update MAY introduce some changes that make the airport card in the Mac more sensitive and this means it may pick up additional routers from other networks (networks you do not plan to connect to) versus what it did before the update. From a computer that is successfully online, you can run iStumbler to survey the routers in your neighborhood. When I did this I found several from other houses down the street that were broadcasting very strong signals. They were on the same channel my own router was on. First, I deleted the existing Location and created a new one. I then had to re-enter the router name and password. Then I went into the more advanced settings for that Location and removed all other networks from the list and told it to only connect to my preferred router. (Leave in names of routers you may connect to at other times, however, such as when you transport your Mac to another physical location.)
I believe that this effectively screened out the other routers whose signals were competing with mine. In fact in one location in a far corner of our house, the neighbor's router is actually stronger than my own, and this was preventing my Mac from consistently staying with my router. After the above changes, it now stays on the proper router, even when its signal is weaker than the others.
From reading the posts, it sounds like while my suggestion may solve SOME of the problems, others have to do with other factors (such as U.S. versus non-U.S. hardware).