OK - who would love to be able to craft a boat out of obsidian (same as the wooden boat recipe, except with obsidian) and sail around on lakes of lava? ME!!!
Not bad, perhaps, but using a minecart still seems like a hack to me and doesn't really make sense. An obsidian boat crafting recipe seems more appropriate, IMO, where wooden boats are destroyed by lava and obsidian boats sink/are destroyed by water. Nice parallelism. Going the minecart route, there would be no reason why they shouldn't also float in water, making boats somewhat pointless...and it doesn't really make sense to me in minecraft logic, if there is such a thing.