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what about Syphone

I don't have a iPhone yet, so I can't try it, but I once came across it and bookmarked it for the future.

I think that serve your purpose.
 
It is but I would not advise restoring a 3G to a 3GS... and if you don't do a restore there is no easy way.

Why wouldn't you recommend this? I've seen several posts here that advise people to do exactly this, yours is the first I've seen that makes it sound like a bad idea. I was planning on restoring my new 3GS from a backup made with my 2G, so if you have any credible sources that point to this being a bad idea I would love to see them!
 
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