Since Heikki doesn't have a blog of his own, I will take it upon myself to blog about this one: he has just committed the initial version of the visibility map feature for PostgreSQL 8.4.
In short, this means that VACUUM will now only need to visit pages in a table that has actually changed. So if you have a large table where only a very few rows change, VACUUM will require significantly less CPU and I/O to run. With this lower cost, it's also possible to run VACUUM more often on large tables - if nothing at all has changed, it's more or less a no-op!
In the future, the hope is to be able to build on this patch to implement things like index-only scans - but that's for another version.
Yet another great feature for 8.4 - this is shaping up to be a very good release I think.
This is the kind of advancements that justify the upgrade worth any adaptation work!
This will be a great release, PostgreSQL will definitely have the most complete SQL dialect amongst opensource databases with CTE and Window Queries!