page is uninitialized --- fixing

We just took a test database down (PG 8.1.11) fairly hard (pulled a SAN
switch out while it was under load), which caused the DB to crash.  It
started up fine, when I vacuumed the DB, I saw the following messages.

WARNING:  relation "my_table" page 652139 is uninitialized --- fixing
WARNING:  relation "my_table" page 652140 is uninitialized --- fixing
WARNING:  relation "my_table" page 652940 is uninitialized --- fixing
WARNING:  relation "my_table" page 652941 is uninitialized --- fixing

That sort of looks like it could be data loss, can someone explain what
happened?

Brad.