Early messages -- in plain Morse code as well as meta-Morse code that translated to written Morse code "beeps", revealed little but what seemed like jokes and internal bootup sequences from GlaDOS. Others were revealed as SSTV-coded images, low-res pictures of random items, all bearing the Aperture Science logo. Within those, fans quickly found a numeric code that turned out to lead to a BBS (remember those?) containing a series of Aperture-related ASCII images of GlaDOS, as well as a series of messages from fictional Aperture Science founder Cave Johnson, who apparently hates both witchcraft and safety.
Steam forum users are feverishly working through all the information to solve the overarching puzzle of what this all means. At this point, being a mere sequel tease, or connecting the Portal and Half-Life universes, would be a disappointment. We already knew there was a Portal 2 on the way, and we already knew that the storylines were loosely connected. Valve hiding a complex series of clues inside an old game is far more interesting than either of those things.
Source: Boing Boing
Source: Steam forums