I would set a keyframe for the object at frame 1 and another at frame 50. Then I'd go back to frame 1 and change global transparency in the material editor to 100. You should now have an incremental shift from invisible to visible.

Edited to add: I realize now that doesn't work, does it? When you set the transparency to 100 in frame 1, it remains transparent throughout. What I did as a workaround was to make 51 frames, with the object solid at frame 1, invisible at frame 2, then visible at frame 51. Render your animation and clip out frame 1. Someone probably has a better way to do it, but that worked for me.