Is the third commandment about not cussing? Well, every ninny knows not to be profane. Perhaps there is a bit more to it. Brevard Childs summarizes the meaning of the third commandment this way:
The initial problem of this commandment turns on its translation. An important ancient tradition renders the sentence: ‘You shall not swear falsely’ (cf. Targum, Syriac, NJPS). Moreover, there is good linguistic evidence to support this translation. The lifting up of God’s name is identical to taking an oath in Lev 19.12 and Deut. 6.13. For most references within the Old Testament the issue does turn on a misuse of God’s name through false swearing. . . .
The heart of the commandment lies in preventing the dishonouring of God. As the source of truth he cannot be linked to falsehood or deception. Nor can God’s freedom be infringed by human manipulation. The third commandment is radically theocentric in focus . . ..
The prohibition against a misuse of God’s name is to be distinguished from a number of other abuses. Outright blasphemy or insults to God which are incurred by foreigners dishonour God and are punishable by death, but are not regarded as oaths (Lev. 24.10-23; II Kings 18.19ff.). The distinction is significant in showing the real area of the commandment’s concern. It is addressed to those within the covenant — in Christian terminology, to ‘believers’ — and the dishonouring of God is a much more subtle danger than outright profanation.
This seems to correlate with Calvin’s interpretation. Sailhamer offers a different nuance (it’s actually the 2d commandment after his reckoning):
God had revealed to Israel his name (Yahweh) and had given to them the corresponding privilege of calling on that name in worship and in time of need. Along with this privilege came the responsibility of honor and respect. Israel was not to call on God’s name ‘for no good purpose,’ that is, they were not to presume upon their relationship with God and think that he was merely at their beck and call. The whole of the instructions regarding the nature of Israel’s worship and the building of the tabernacle (Ex 25-31) was intended to teach Israel the proper way to call on God’s name.**
*Brevard Childs, Old Testament Theology in a Canonical Context (Philadelphia: Fotress, 1985), 68-69.
**The Pentateuch as Narrative (Grand Rapids: Zondervan, 1992), 286.