"Rock Am Ring" (Nürburgring Race Track) is Germany's biggest festival and definitely an option - and yes, Bowie said at the Berlin show that he would be back in spring . "Rock Am Ring" was planned for the May, 30th, 2004 but they had to give way to the Formula One race that is there on the same weekend. No news of a new date yet as far as I know.
"Rock Am Ring" is usually twinned with "Rock Im Park" (Nuremberg) meaning that a band plays one day at the Ring and the next day in Nuremberg and vice versa. I think it was Neil Young who once even played both stages on one day/night. The "Rock In The Park" date is still the weekend of 28.-30. of May, so maybe we'll have Bowie there. Sure as he seems about his reappearance here in spring I'd rather bet on this show than on the iffy "Rock Am Ring".
Another option would be the less big (40.000) but far cooler "Hurricane 2004" festival in Scheessel (Hamburg/Bremen/Hannover area). Last year's event was headlined by Radiohead, Björk and Coldplay. So he might rather be with those than the likes of Metallica and Iron Maidon.
And I want to believe
In the madness that calls 'now'