Stack Overflow should allow iphone-3G tag, however since the iphone-3gs tag exists, it cannot be created / used.
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I added your requested tag to this question, so now it exists. Go wild and free with it.
Note that I don't think the existence of the iphone-3gs tag was preventing you from creating it (assuming you have sufficient rep to create tags). All you have to do is dismiss the tag dropdown and enter the new tag manually.