we (Songkick) have already built an aggregator for concert tickets - covering 17 ticket vendors in the UK and US. If you're looking for gig tickets I'd love your feedback. Thanks
Originally, the plan was to stick to Sports tickets for at least a year and then maybe add concerts (if at all). However, right after we launched, despite having "sports" and sports leagues plastered all over our site, more than half our search logs were concert related. Additionally, numerous publications said they would cover us if we had concerts and we received email after email demanding concert and theater listings from our users.
I was actually price hunting for Radiohead tickets for tomorrow in Toronto. ;) I think they are a bit out of my pricerange everywhere, the worst tickets are selling for twice as much as I scalped my extra, amazing tickets for Philly.