I manage to find this out yesterday. Here are the steps i took to change the slider.
1) Download any lock slider you like.
iPhone Forums at ModMyI.com - iPhone | iPod Touch, news, apps, themes. - Downloads - Lock Screens & Sliders
2) Create a folder. e.g: Macslider
3) In Macslide, create another folder and name it as Folders.
4) In the Folders, create another folder and name TelephonyUI.framework.
So you should have MacSlide/Folders/TelephonyUI.framework.
Then, put the downloaded sliders to the TelephonyUI.framework.
In TelephonyUI.framework, u should have the slider png files.
*bottombarknobgray.png is for the bottom unlock slider "slide to unlock".
*bottombarknobred.png is for the top slider "slide to power off"
SSH the theme to Library/ Themes/
Select the newly created slider theme via Winterboard and respring.
You will have your own custom slider.
Hope this helps.
Cheers