What's the deal indeed...I'm not sure there is one. Justin seems to think so. He's made it into this month's session* topic. As with all beer styles its a continuously shifting entity that changes with the advent of new brewing techniques and different hop flavours. It is one of the more wide ranging categories ranging from the oxymoronic black IPA to the wheat heavy white IPA. Yes you could argue that they're a subset of hoppy porters and wheat beers but that's missing the point, the brewer set out to brew a beer with the hop complexity of an IPA...in taste they're largely still IPAs but they're not IPAs because they're not pale...
One new "style" that doesn't seem right is "session IPA" because IPA doesn't imply strength necessarily ; just look at some of the longer running UK examples (though the fact that these aren't hop-forward doesn't stop them being called IPAs either). Sessionability means different things to different people, I like Boak and Bailey's ideas.
IPA is no longer a tightly defined style, but I think we as drinkers benefit from that. Who wants every beer to taste largely the same? Of almost 1700 beers sampled in the past 2 years (and countless more before that of course) 242 were members of the IPA family, (that's 14% fact fans) and a whole range of beers from top 10s right down to down the sink duffers. And new beers continue to surprise me all the time. What I'm trying to say is what the beer is called shouldn't matter, its what's inside the bottle that counts. The name IPA just gives people an idea of roughly what might be inside the bottle. Its so popular because its not just a single beer style any more.
*If you don't know what The Session is you must have been asleep for the last five years. Details over at the Brookston Bulletin. Yes, I'm a little late this month...it crept up on me. I've not posted a response for a while, largely because the topics have been uninspiring (no offence to the hosts!) but this month's got my words flowing again so cheers Justin!