it appears the problem was the fiber link between the two sites. I'm sorry I didn't update this post any sooner, but somehow I don't get email updates from every post I'm interesed in and this one slipped my mind.  So the problem is solved by using a different long distance fiber.
I've got 3 Brocade switches per fabric, 2 on site 1 (5100 + 5100) and another one (4900) on site 2. The two switches on site 1 are ISL'd over MM 50/125 and working fine, but the triangle connecting these two to the third switch doesn't work. Or in fact: the 9 micron LW SFP's don't see any light.  We changed patch cables, used a different line from site 1 to site 2, we set the SFP's to fixed 4Gb, but still: no light on the LW SFP's.  We have 1 Brocade 4900 and two 5100's. The 4900 has a 4Gb LW SFP and the 5100 are using 8Gb LW SFP's.  Could it be an SFP incompatibility ? Connecting the two 5100's over LW works just fine.  The distances over the two links are 330m and 580m (really close by) and we really need the 4Gb link, otherwise we'd have used 2 or 1Gb ISL's. We are using 5dB attenuators on both loinks to damp the power. We tried leaving the attenuators out and we tried stacking the attenuators (so 10dB), but nothing helps.  Does anyone have any idea why we don't get any light between the two sites ? Tips ? Things we need to look at ?
Problems connecting 2 Brocades over LW 9/125