Mostly harmless
In the Re-imagining
Earth is the alleged location of the 13th Colony, destination of the 13th tribe of Kobol, as related in the Sacred Scrolls.
Commander Adama uses the legend of Earth to give his people hope following the devastating Cylon attack on the Twelve Colonies, claiming that it is no myth and that its location was a closely-guarded secret among the senior military leaders - himself included.
While she is initially sceptical - going so far as to rebuke Adama for his lies to his crew, Laura Roslin nevertheless becomes the most fervent believer in Earth. This transition is apparently due to her use of the hallucinogen Chamalla to treat her breast cancer, whose side-effects she attributes increasingly to prophetic vision. For Roslin, these begin with a dream about Leoben Conoy before he is discovered hiding in the fleet (Flesh and Bone) - and who first gives them a clue that they will find Earth; and they culminate with visions of the Tomb of Athena on Kobol and the Arrow of Apollo (Kobol's Last Gleaming, Part I), both of which set her at odds with Adama, and result in her arrest and removal from office on Adama's orders (Kobol's Last Gleaming, Part II).
In Home, Part II, Starbuck quotes the Sacred Scrolls and says that Earth is the place where the people of the Thirteenth Tribe could look into the sky and see their twelve brothers. Judging from her expression, the interpretation of this passage had previously been ambiguous, at least until the events of the episode made the literality of the passage clear.
In the Original Series
Earth is the Thirteenth Colony; its settlers having departed long ago from Kobol after the same environmental cataclysm which caused the Exodus of the Twelve Tribes and made Kobol uninhabitable. As stated in the Book of the Word, the Thirteenth Tribe left opposite the other Twelve Tribes, the information of their destination residing in the tomb of the Ninth Lord of Kobol. (Lost Planet of the Gods)
Transmissions from Earth -- i.e. the Apollo Moon Landing -- were picked up by Captain Apollo in the Celestial Temple on an obsolete Gamma Frequency, though the copy of the transmission was destroyed in the attack on the Cylon basestar. (The Hand of God)
Notes (Non-Canonical)
In the nefarious Galactica 1980, Earth was discovered. Much to the dismay of Adama and the survivors, the humans on Earth were underdeveloped and would not be able to defend themselves from the Cylons.