I would like to know if it would be possible to replicate the effect like the bottom of the Top Tweets list with pure CSS?
Yes you can! Taking advantage of RGBa colors and CSS3 gradients, we can apply the following styles to an element and have a fading semi-transparent background:
Mozilla:
background: -moz-linear-gradient(top, rgba(255,255,255,0), rgba(255,255,255, 1));
Webkit:
background: -webkit-linear-gradient(top, rgba(255,255,255,0), rgba(255,255,255, 1));
(Updated after changes to Webkit gradients)
Sadly, this only works in Firefox 3.6+, Safari, and Chrome. If you need the effect in IE or older versions of Firefox, then you'd be better off using the semi-transparent PNG like Twitter does.