According to THQ UK, Sonic Battle will be released on the 27th February ’04.
On a fairly unrelated note, it appears that Sonic the Hedgehog the Movie (the Sonic Anime) will be re-released on DVD on the 15th December ’03 (According to Amazon.co.uk, Play.com et al).
Source: THQ UK (Sonic Battle), Various online retailers (Sonic the Movie)