[The Saucepan Man]

I imagine his spirit is just wafting around ME--I mean he can't go back to Valinor, he isn't cast into the void like Melkor....

In The Complete Tolkien Companion by JEA Tyler, it is said that, upon the destruction of the One Ring, Sauron "was cast into the void for ever and the fear of his domination was lifted from the World". Clearly that's not a primary source and it may well be wrong, but I do seem to recall reading some basis for it in Tolkien's works. I just can't recall where. Has anyone else come across anything which might suggest that Sauron was in fact cast into the void upon the Ring's destruction?

I say this; he took form as the necromancer, physical form. That dose not explain why he did not take a form when he returned to Mordor?

In The Letters of JRR Tolkien (which I am currently reading and would heartily recommend), JRRT said (in a footnote to Letter no 131):

It is only in the time between The Hobbit and its sequel that it is discovered that the Necromancer is Sauron Redivivus, growing swiftly to visible shape and power again. He escapes the vigilance and re-enters Mordor and the Dark Tower.

The references given in the link above confirm that Sauron did have physical form during the War of the Ring. But this letter suggests that he was not able to assume such form while residing incognito as the Necromancer of Dol Guldor.

As for the original question:

If the ring of power was destroyed, would not the power that Sauron put into then revert back to him?

Tolkien yet again provides the answer in Letter no 131 (this letter is essential reading for anyone interested in Tolkien's works as it provides the answers to so many questions frequently raised in this forum). He said, in reference to Sauron's "Ring strategy":

But to achieve this he had been obliged to let a great part of his own inherent power ... pass into the One Ring. While he wore it, his power on earth was actually enhanced. But even if he did not wear it, that power existed and was in 'rapport' with himself: he was not 'diminished' ...

There was another weakness: if the One Ring was actually unmade, annihilated, then its power would be dissolved, Sauron's own being diminished to vanishing point, and he would be reduced to a shadow, a mere memory of his malicious will.

In other words, the Ring represented a large part of Sauron's power - power which he could draw on even when the Ring was not in his possession. But when it was eventually destroyed, that power was destroyed with it and the residual power left to Sauron was so little as to effectively render him 'powerless'.