Actually, they already have some sort of 'partnership'.
If you join the Facebook Group 'Ticketmaster' - iTunes gives you 5 free downloads from the iTunes Store.
I think its more likely they have formed a partnership, rather than staged a war - if what I have written above is anything to go by.
PS: It's not just hear say, I have joined the group and got my 5 free downlaods. When you enter the code into iTunes, it even brings up a Ticketmaster logo, and says thank you for joining the Ticketmaster Facebook group. This means Facebook/Ticketmaster aren't just dishing out any ol' store codes, as iTunes recognises them as origining from Facebook - even displaying the logo of the company etc.
Therefore, I think a partnership is more likely.