Last week Microsoft unveiled the new version of its Windows Media Player firing the opening shots in a long-anticipated battle against Apple Computer for supremacy in the online music business. Both companies are targeting the fast-growing market whose sales are expected to be $270 million this year but could grow to $1.7 billion by 2009. Experts at Wharton and elsewhere say that for now Apple whose iTunes music service commands a 70% market share has an impressive lead. Over time however two strategic issues make Apple vulnerable to being dislodged by Microsoft or other rivals.