It's possible that a new iPhone will be needed to fix the antenna issue, but there's no guarantee it's coming. If the new iPhone isn't announced in September (for an October release), then it's likely to be in January for a Feb. release. But if it isn't in February, then it'll be in June. Short version is that who knows when it's coming.
So I'd say that if your current one works fine, then you should wait for Feb., as that seems to be the most likely time a new iPhone is available, according to rumors. IF there's still no new iPhone, the 4 isn't that bad, so just go ahead and get it then if you're impatient.