Is the battery replaceable?

Eventually? Yes.

As with every other iPhone, the battery is enclosed inside the case. Apple's intention is that the battery should not be replaced by end users. In practice, as iFixit's iPhone 6 Teardown illustrates, the battery is socketed, and fairly easy to remove once the display is removed. In the future, as with every previous iPhone, there will be aftermarket replacements for the iPhone 6 and 6 Plus batteries, so that DIY users/repairpeople can replaced failed batteries.

But until the Chinese battery OEMs get their replacement channel going, the only source of replacement batteries for the IP6/6+ is Apple. Why would Apple sell you a replacement battery for a phone that's still under warranty? If you want to keep the warranty valid, Apple (or Apple-certified repairpeople) need to replace that battery.

In today's situation, no. At this stage (five days after the release of the iPhone 6), there is absolutely no reason for a rational phone customer to disassemble their shiny-new iPhone 6 and void their warranty to extract a battery that's supposed to work perfectly fine. If the original battery doesn't work, the owner should yell at Apple to replace it, which will keep their warranty intact.

Just as with every previous generation of iPhones, aftermarket repairpeople/shops are just going to have to accept that they won't get many current-generation iPhones coming in until the warranties get close to expiration. The only IP6/6+ repairs you're going to get for most of the next year will be for things like smashed displays, where the damage has already voided the warranty. This shouldn't surprise you; isn't this what's happened for every previous iPhone?

I realize I'm making a big deal out of the Apple warranty (or the AppleCare extended warranty, if you bought that). The price of the basic warranty is built into the original purchase price, and the AppleCare warranty was additional money; if Apple's promised to repair your equipment, and you've already paid them to repair it, shouldn't you get what you paid for? Besides, if Apple sees the things that fail, they may possibly design the next products so that they don't fail the same way.