They do need a fair bit of force. Best way for me was first finger and thumb underneath, either side of the post; and first finger of the other hand on top to prevent it "jumping". Then just move up in one smooth motion.
Note on re-assembly - there is a "pin" formed from the top of the internal frame which needs to go through the corresponding hole in the antenna assembly. Although the antenna will clip down without it being exactly in place, the antenna will not be "sprung" and will probably foul the cover when it is replaced, probably causing damage.
As you almost certainly wouldn't notice this as you replace the cover, do check that this "pin" is aligned and that the antenna is floating on the spring, before replacing the cover.
On my late 2009 2.53GHz C2D, there's no tape holding the antenna wire to the frame but it *does* sit in a "clip" formed off part of the internal frame. There's very little slack in the wire for this BT antenna and so it pays to drop the wire back into the "clip" and re-insert the antenna in one motion.
I second the comment about trapping wires on reassembly, particularly the WiFi one. The cable's natural tendency is to trap itself between the black plastic airduct section of the internal frame and the corresponding moulding on the base / port side.
In the absence of a third hand, I found that resting the WiFi antenna on top of the optical drive as you lower the internal frame back into position helps position the cable in the right place, rather than leaving the antenna outside the port area.