There've been enough posts about this that I thought a separate solution thread was in order...
If after upgrading to 1.5 you find links to *static* Pages, specifically when using custom permalinks, cause 404 errors, you want to:
1. Copy the contents of your .htaccess file to a file on your pc.
2. Edit out all .htaccess contents and save it through Manage > Files.
3. Reset your permalink structure (Options > Permalinks, click Update Permalink Structure).
In most cases .htaccess will be written to automatically. If not, copy the rewrite code WordPress generates to a text file and upload that as your .htaccess file (or edit it online -- Manage > Files).
Any custom (non-WP generated) rewrite rules you may have had can be copied into .htaccess, but I recommend adding them to the end of the file (after
# END WordPress).
Further information on editing .htaccess can be found here (when the link is working again):