Yes, the forums_search_delta was dropped, and the two other ones created, in umm.... one of the point releases. Don't remember which one, off the top of my my head. If you're running v3.0.5, I know the following will work since it's what I do on my dedi.
Remove all instances of forums_search_delta and replace with the two new ones in your config files and cron jobs. Replace /xxxxx with /your-user-account-name, if applicable, otherwise remove that part of the path. Here is a typical cron pair, taken from an install of mine:
*/15 * * * * /usr/local/sphinx/xxxxx/bin/indexer --config /usr/local/sphinx/xxxxx/etc/sphinx.conf core_search_delta forums_search_posts_delta forums_search_topics_delta members_search_delta downloads_search_delta gallery_search_delta calendar_search_delta blog_search_delta --rotate > /dev/null 2>&1
0 3 * * * /usr/local/sphinx/xxxxx/bin/indexer --config /usr/local/sphinx/xxxxx/etc/sphinx.conf --all --rotate > /dev/null 2>&1
NOTE: The above jobs are for an install with all core applications: Board, Blog, Downloads, and Gallery. REMOVE any that are not applicable to your install (e.g. blog_search_delta, downloads_search_delta, gallery_search_delta).
Hope that helps.