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The underside of the alleged iPhone 5 chassis shows some significant differences. Along with the addition of a headphone connector (moved from the top) and a larger speaker grill, there's also a significantly smaller dock connector hole.
Rumors relating to this revamped dock connector suggest that Apple is finally ditching the existing 30-pin connector for something smaller. A 19-pin, 16-pin and even 8-pin connector has been rumored, although no evidence for any of these has surfaced.
It has also been rumored that the new connector will be magnetic, similar to the MagSafe connectors that Apple uses on MacBooks.
If the dock changes then this could mean that all current iPhone accessories are rendered obsolete overnight unless Apple makes the new dock backwards compatible with the old dock.
Image source: ETradeSupply.