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One of the hidden benefits of the 3G iPad was improved GPS accuracy and speed over the iPhone 3GS. The improvement was attributed to a newer GPS chip from Broadcom. iFixit's teardown revealed that the iPhone 4 shares the same Broadcom BCM4750 chip as the iPad, suggesting the iPhone 4 will see the same benefits.
At least one reader experience confirms the improved GPS accuracy and speed of the iPhone 4:
I used my Navigon GPS app today for about ten minutes, nothing to long or in depth, but the [iPhone 4] got a lock in seconds and was spot on. I'll test it tonight going home from work and see how it does but I didn't notice any issues earlier today, in fact it was a huge leap from the 3GS in terms of reliability and accuracy.