A new Cydia tweak by Itay replaces the "." that is automatically inserted when you press the spacebar 2 times. Custom Double Space enable the "double space" command to automatically insert a custom text string instead of the iOS standard period. This is free Cydia tweak is a nice customization to have the ability to set.
You can also set this tweak to insert a "double space" to disable this automatic function if it is irritating you. If you aren't running Cydia on your iPad, then you are using it to it's full potential!