[Boromir88]

I believe so Eomer...Saruman was playing a dangerous game. Saruman wanted the Ring, power, and control for himself. And I think the easiest way for him (in his mind) to get that would be to join with Sauron. It seems like a rather smart thing to do, I mean militarily the people opposing Sauron had no shot at all. So the easiest way to get what Saruman (from Saruman's POV) would be to actually join with Sauron and then betray him.

Of course these were Saruman's 'secret plans' and as he tells Gandalf; 'We can bide our time, we can keep our thoughts in our hearts...' (The Council of Elrond). Despite this I believe Sauron figured out what Saruman was up to, and Saruman really had not kept his thoughts hidden:

'I Grishnakh say this: Saruman is a fool, and a dirty treacherous fool. But the Great Eye is on him.'~The Uruk-hai

'But they shall help to rebuild Isengard which they have wantonly destroyed, and that shall be Sauron's, and there his lieutenant shall dwell: not Saruman, but one more worthy of trust.'~The Black Gate Opens

Just as some interesting side notes. When Denethor's father (Ecthelion) was the Steward; Denethor didn't like Gandalf and kept telling his dad he should trust Saruman...and in an early draft (Home VII The Treason of Isengard) Tolkien played with the idea that Boromir would side with Saruman. So, there are some loose connections with a Gondor-Isengard relationship; however once Saruman turned to evil, I doubt Gondor would have any interest in siding with him.