World of Apple says March 24 is the possible date for a refresh of the entire Apple's desktop line. We don't have confirmation of this yet, so as always, remember our rule about rumours.
Never trust them.
The rumour doesn't have any details as to what could happen, so if true, we can only speculate for now.
The Mac Pro and Mac Mini are long due for a redesign. As part of PowerPC-to-Intel migration, all the computers kept their old designs during the first phase of the transition to the new hardware architecture. The idea was to maintain a sense of continuity. Then, as the Intel-based Macs proved to be a success, Apple started to change external designs. The Mac Pro and the Mac Mini are the only two computers that have not had a external redesign yet. [World of Apple]