In fact those borders are actually translucent lines. The issue comes from OpenGL's way to render the quads and is "fixed" in OL2.0 so that the borders don't appear.

To fix that you could make the Bitmaps one pixel wider and high than supposed (by copying the last row and column) and then rendering those in a reversed order.

An another (and better) way would be to compile OpenLayer again with the fix, but unfortunately I've split the image loading and rendering to several parts behind the scenes to allow OpenLayer to work easier with SDL, for example. I've changed so much that I don't remember which changes were important for the fix anymore... This is the relevant code and should be placed in the Load-methods right before SendToGPU. It might not be enough, though.

PS. I'd be better if 2.0 would be ready already, but it doesn't still feel complete to me. And to consider that originally it was only going to be a really small update...

EDIT: Tried to make it clear that the old Bitmap API isn't going to change.

The problem is that the last row and column are both translucent because the invisible translucent parts in the right and bottom side of the Bitmap are blended with the opaque pixels in the edges (or so I think):

 _____________
|visible |    |
| image  |    |
|________|    |
|  invisible  |
|   borders   |
|_____________|

A Bitmap with exaggerated borders

Those borders exist to make the dimensions of the Bitmap powers of two. Some hardware support non-power-of-two images and those are supported in OL2.0 (once again )

However, in usual hardware when you render the image at subpixel coordinates the borders of the image aren't clipped properly if there's borders around the image.

To prevent that, make the Bitmap one pixel wider and higher than it's supposed to be. Then render the tilemap so that you can't see any overlapping - thus you need to render the tilemap from bottom to up, right to left.