Shopping for iPad speakers doesn't have to be complicated. If you already have a speaker system you like, you're probably just a .99-cent stereo aux cable away from making it work with an iPad (or anything else, for that matter).
Generally speaking, there are four ways to get sound out of the iPad. It can go out through the headphone jack to anything, though you run the risk of distortion if you're plugging into a system with another gain stage (such as a car stereo aux input). For a cleaner signal, find a system that plugs into the iPad's dock connection. Typically these will tap the iPad's line-level output, or perhaps its digital out, and may even work to charge the device.
Your other two options are wireless. The iPad can stream to any A2DP-compatible Bluetooth speaker--almost any stereo Bluetooth speaker will fit the bill. There are tons of these speakers on the market, but only a handful that deliver decent sound. Bluetooth speakers also happen to be some of the most portable options available, with examples like the Jawbone Jambox.
Finally, there's a new breed of speakers using Apple's AirPlay system for streaming music over your local Wi-Fi network. Like Bluetooth, AirPlay speakers are wireless, but because they use your network's Wi-Fi bandwidth, there's no loss in audio quality from end to end.
There are only a handful of AirPlay-compatible speakers promised so far, but with the success of compatible devices, including the iPad, iPhone, and iPod Touch, more are sure to come. Until then, you also have the option of connecting an Apple TV or AirPort Extreme Base Station to your existing system as a relatively inexpensive AirPlay bridge.
To see our favorite picks, check out CNET's top iPad speaker roundup.