NOTE*: There is now a Windows version of this tutorial here: How to Make iPhone SMS Alert Vibrate Longer Using Windows
Click to launch Fugu from the dock.
Enter your iPhone's IP Address in the Connect To: field. Enter root into the Username field then click the Connect button.
If you do not know you iPhone's IP Address you can find it here:
How to Find the IP Address of Your iPhone
If prompted enter your password which is usually alpine or dottie then click the Authenicate button.
Once Fugu logs into your iPhone click the Go To: button from the toolbar.
Make sure Remotely is selected and enter /System/Library/Frameworks/Celestial.framework/ into the Go To field. Then click the Go button.
Fugu will go to the Celestial.framework directory. Drag SystemSoundVibrationPatterns.plist from the right pane to the left pane.
Hold down control and click the file (SystemSoundVibrationPatterns.plist) once it appears in the left pane. Select Open from the popup menu that appears.
Property List Editor will open SystemSoundVibrationPatterns.plist. Expand Root then Default so that you see OffDuration, OnDuration, and TotalDuration.
Change OnDuration to a longer value. I set it to 1.4 instead of .4, increasing it by 1.0. Then add the same amount you added to OnDuration to the TotalDuration number. I set it to 1.5 increasing it by 1.0.
A setting of 1.4 will make your phone vibrate for around 1.4 seconds.
Select Save from the File Menu.
Now back in Fugu drag SystemSoundVibrationPatterns.plist from the left pane to the right pane.
It will ask you to confirm the overwrite. Click the Overwrite button.
Reboot your iPhone to see the changes take effect !
A big thanks goes out to one4house, fabiopigi, and Snowbird for figuring this stuff out!
Windows users: You can't open plists properly unless you convert them to text. You can use this link(Thanks Snowbird) to do so: http://188.8.131.52/~khchung/cgi-bin/plutil.cgi. Use WinSCP to get the file then convert it to text using the link above. You can then modify it and upload it to your iPhone. If its not clear I can make another tutorial for Windows users. Leave comments...