While I do like the idea of having an easily exchangeable battery, it's not really that hard to change the PSVita battery. Just look at the iFixit help. Voiding warranty on the thing is not really the issue for most since modern batteries should last a bit longer than the older ones and thus the lifetime of your warranty (1-3 years for most devices, probably 1 year). If it does break during the warranty period, you would just send it to the manufacturer, regardless of user friendly battery removable solutions. At least I would.
As some of you seem to be very keen to condemn Sony on the matter, do note that the battery is very easily taken off of the devices back plate. If it was meant to be impossible to remove, in other words only in Sony's own repair, they'd have gone the Apple route and glue the fucker into the base.
TL;DR: Removing the battery is inconvenient, but not impossible. You shouldn't change the battery anyways during your warranty period. It's not the end of the world.
As some of you seem to be very keen to condemn Sony on the matter, do note that the battery is very easily taken off of the devices back plate. If it was meant to be impossible to remove, in other words only in Sony's own repair, they'd have gone the Apple route and glue the fucker into the base.
TL;DR: Removing the battery is inconvenient, but not impossible. You shouldn't change the battery anyways during your warranty period. It's not the end of the world.