It can void your warranty, so if you can afford to lose an iPhone, then by all means.
An itunes card or pre-paid visa on the other hand won't void your warranty.
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iPhones have not been working well with the new ios5, especially the older models like the 3gs. If the jailbreak application locks up mid way you would have bricked the phone.
Many for cost apps have a 99% functional no cost application. For example the gps street map app TomTom costs $40.00 (usd) but the mapQuest application is free. the mapQuest app works well, lasts a long time.
there should be 2 parts to a jailBreak app. The second part should restore your previous operating system back to apple standards. JailBreak apps have been around since the iPhone came out, look for one with good reviews.
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