There are a lot of annoying things about hydration packs, but for me filling up isn't one of them. Apparently for someone, it is. UK hydration company Source has built the Universal Tap Adapter (UTA), which allows you to fill your bladder without removing it from the backpack. The system has a replacement hose (which may or may not work with non-Source bladders) with a one-click removable bite valve. After pulling the bite valve off, simply attach the adapter valve and fill up from the sink or a water bottle.
Its $35 price tag seems kind of expensive for the small convenience of filling your bladder without taking it out, but it's an interesting little gizmo for anyone that finds filling up existing bladders a hassle. Source Hydration