Solution (Tested Personally) :
To get Wi-Fi fixed on your Samsung Galaxy I 9000 follow the instructions detailed below :
- Make sure Wi-Fi is ON and Navigate to Settings -> Wireless and Network -> Wi-Fi Settings, now click the left-side context-menu button on your phone to bring-up Advanced option and click it.
- Now to get things working we will need to make cellphone use a static IP address for Wi-Fi, check the Use Static IP option and proceed with entering values manually.
- If you are not sure what values to use take a look at your network-adapter status on computer, here are my settings (should works on most default router setups).
- Enter all values and your screen should now look like the one below.
- Now go back to your Wi-Fi settings page and click your access-point name and it should show you the manually entered IP value.
- Your Wi-Fi issues should be now over.
If you need help with manual values do ask here.