Even if you mod-rewrite your session ids away (mypage.php?id=5 --> mypage5.htm) your problem still isn't solved: either the SE will pick up lots of identical pages with different names which might put you into trouble. Or they decide not to pick them up at all.
My suggestion: don't use any sessions / session id in public places where spiders drop by. Only use sessions for shopping carts, etc. where spiders are off-limits.