Conjurations
Powers that have the conjuration keyword create
objects or creatures of magical energy.
Unless a power description says otherwise,
a conjuration cannot be attacked or physically affected, and a conjuration does not occupy any squares.
A conjuration uses your ability scores and defenses
to determine the outcome of attacks it makes and
attacks against it (if such attacks are possible).
Environmental phenomena and other forces have
no effect on a conjuration unless a power description
says otherwise. For example, a conjuration that produces
an icy hand functions in a fiery, volcanic cavern
without penalty.
If a power allows you to move a conjuration, at least
1 square that the conjuration occupies must remain
within the power’s range. If you move far enough away
from a conjuration that it is no longer in range, its
effect immediately ends.
If a conjuration’s creator is slain, the conjuration
immediately ends.
The flaming sphere specifically states, (in updated form) that it occupies a square. It does not state that it is passable by anyone, allies or enemies, such as the cleric power Knights of Unyielding Valor allows allies but not enemies (interestingly, the cleric is not his own ally, so he cannot pass through or gain cover from his knights, though I would errata this), and the cleric power Astral Defenders allows creatures (the caster, his allies, and his enemies) to pass through.
If the conjuration occupies a square, then it's impassible unless it specifies otherwise. If it doesn't occupy a square, then it's passable unless it specifies otherwise (wall of ice). As an impassible square, it is impossible to force another creature into the conjuration.
Overall, conjurations are a weak spot in the rules, and have inconsistent language. WotC needed to tell a guy to read every description of every conjuration and rewrite each power so that they all read the same. I hope that in all future books they all have the same language.