Good question - there has been much debate on this and frankly your guess is as good as mine. Another example of ambiguity in the Qur'an imho.
But this is what wikipedia says:http://en.wikipedia.org/wiki/Sabians
The recent debate on who the Sabians were is directly connected to how to best translate the following verses from the Qur'an out of the original Arabic. The Qur'an briefly announces the Sabians in three places and the Hadith provide further details as to who they were as people of the book:
"Verily! Those who believe and those who are Jews and Christians, and Sabians,  whoever believes in Allāh and the Last Day and do righteous good deeds shall have their reward with their Lord, on them shall be no fear, nor shall they grieve ." Quran 2:62
"Those who believe (in the Qur'an) those who follow the Jewish (Scriptures) and the Sabians and the Christians― any who believe in Allāh and the Last Day, and work righteousness―on them shall be no fear, nor shall they grieve." Quran 5:69
"Verily, those who believe, and those who are Jews, and the Sabians, and the Christians, and the Majus, and those who worship others besides Allāh, truly, Allāh will judge between them on the Day of Resurrection. Verily! Allāh is Witness over all things awitness." Quran 22:17
The Sabians existed before Muhammad, and are said to have read from a book called the Zabur ("Psalms"). They came under Islamic rule about 639 AD. At that time in history they were described as Greek immigrants but were grouped together with the Nabataeans.
Many Islamic writers from the period of about 650 CE onward gave further descriptions of the Sabians. They wrote that the Sabians lived in Iraq around Sawad, Kutha and Mosul and they "wash themselves with water", had "long hair", and "white gowns". They had a monotheistic faith with religious literature (the Zabur) and acknowledged the prophets. Their theology resembled that of Judaism and Christianity yet were neither, nor were they Magians.
With regard to their beliefs, Ibn al-Qayyim said: "The people differed greatly concerning them, and the imams were unsure about them because they did not have enough knowledge of their beliefs and religion." Al-Shaafa’i said: "Their case is to be examined further; if they resemble the Christians in basic matters but they differ from them in some minor issues, then the jizya is to be taken from them. But if they differ from them in basic issues of religion then their religion cannot be approved of by taking the jizya from them." And he elaborated elsewhere: "They are a kind of Christian." consistent with a comment about some of them mentioned in Bahá'i writings.
Ibn al-Qayyim said: "The Sabians are a large nation among whom are both blessed and doomed. They are one of the nations who are divided into believers and disbelievers, for the nations before the coming of the Prophet (Peace and Blessings of Allāh be Upon Him) were of two types, kāfir nations all of whose people were doomed and among whom were none who were blessed, such as the idol-worshippers and the Magians; and others who were divided into those who were blessed and those who were doomed, namely the Jews, Christians and Sabians."
According to Islamic scholars, they did not reject the Prophets of Islam but neither did they regard it as obligatory to follow them. Whoever followed (the Prophets) may be blessed and saved, but whoever follows a path similar to that of the Prophets by virtue of one's own reasoning is also blessed and saved, even if one did not follow the Prophets in specific terms. In their view the call of the Prophets was true but there was no one specific route to salvation. They believed that the universe had a Creator and Sustainer, Who is Wise and above any resemblance to created beings, but many of them, or most of them, (i.e. the Sabians of Harran) said: we are unable to reach Him without intermediaries, so we have to approach Him through the mediation of spiritual and holy Bud Asaf who are pure and free of any physical elements and who are above place and time, rather they are created pure and holy.
‘Abd al-Rahman ibn Zayd (d. 798 AD) wrote: "The Sābi'ūn say that their religion is a religion to itself and they live near Mosul (jazirat al-mawsil) and believe in only one God." He also wrote that they have: "…no cult though their main belief is 'La ilaha il Allāh'." He also remarked that: "the Sābi'ūn did not believe in the Prophet Muhammad (in the same way as his followers did), yet the polytheists were known to say of the Prophet and his companions 'these are the Sabians' comparing them to them." following the Din of Noah as a sect who read the Zabur akin to Christianity. They appear to be between Judaism and Magianism but are in fact closer to Judaism. Sābi'ūn recognise the practice of the prophet Muhammad in going to the caves prior to his inspiration, as in accordance with the Sabi quest for Tawheed Hunafa' and, in general, many similarities with the Sabians meant Muhammad and his companions were often considered to have been Sabians. Most specifically this was because of the Sabian shahada “La ilaha ila Allāh”.
The root-meaning of the word Sabi (deriving from their religion Seboghatullah) means Proselyte, and is identical in usage with the Greek words Sebomenoi or Theosebes and to a lesser extent Phobeomenoi.