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Apple says the Touch was meant mainly to present typical iPod features, not to replicate the iPhone, and it included the Web browser only so users could get onto Wi-Fi to use the mobile music store in certain places that required a log-in screen.
But it seems ridiculous to me to sell a powerful device with Wi-Fi and a huge screen, and to leave out things like an email program, even though you can use Web-based email programs. I assume Apple was concerned that the less costly Touch might compete too much with the iPhone if it had these features. In fact, if somebody can jam a voice-over-Internet capability into the iPod Touch, it might be more of a threat to the iPhone, which is tethered to a single cellphone carrier, AT&T.