According to Blabbermouth and Bill Ward's website, it will be a full-on Black Sabbath reunion at Ozzfest after all:

Dear Sabbath People,

I have communicated to Sharon that I will accept the proposal of 5/11/04 regarding me playing Ozzfest, and today I received a very positive response from her. At this time, I am taking this response as a move forward. This announcement is for the benefit of all those who these past few days have been so outspoken favouring and supporting me as a member of Sabbath. I personally thank all of you. To All Sabbath People On Earth - Rock & Shine.

-Bill Ward
5/20/04

His site also cautions:

Please be aware that this has not happened yet. This statement just means there are discussions, and while said discussions might lead to Bill being on the Ozzfest tour, it has not happened yet.

If he said he'll accept whatever proposal's been put forth by Sharon, I'm VERY confident that it'll happen. A Sabbath reunion isn't a reunion without all four members, so I'm sure it'll happen.