Today - yes, but not with anything Sling Media sells.  If you're laptop has a video out, you can connect it to the TV directly.  If you have a TiVo, you can convert the video to the correct MPEG-2 format and upload it with TiVoToGo.  There are a number of Media Center Extenders that will allow you to play video from a PC over a network - including the XBox 360, and AppleTV.  Etc.

Later this year, Sling Media will release the SlingCatcher, which is a device that will connect to your TV and allow you to stream media from your PC/laptop, or a Slingbox, to the TV.

-MegaZone, GizmoLovers.com
Slingbox PRO-HD w/TiVo Series3, Slingbox SOLO w/TiVo Pioneer DVR-810H, SPM Treo 680, SP WinXP
(Former Sling Media Beta Program Manager.)