Sorry to revive such an old thread, but the new books do shed some light on the Arcturians.
Spoiler
River of Pain which is set a little before Aliens states that Weyland-Yutani has had Arcturian trading partners and that humans have met some other intelligent beings, some hostile and some friendly.
Sea of Sorrows which is set towards the end of the 25th century says that the Arcturians were the first intelligent race to have been discovered by humanity, and these encounters with other races inspired Weyland-Yutani and other companies to focus on technological advancement to make sure humans don't lag behind, maybe this trading with Arcturians helped.
When new alien ruins are discovered on LV-178, it is said to be the greatest discovery made, as great as the discovery of the Arcturians.
And now in the new Rage War trilogy, Tim Lebbon teases that they'd be mentioned again.
Another author said that there are more plans for them, I don't know what they have in store for this elusive species.