I'm quite certain this was working fine previously with earlier builds... not sure exactly when it broke.
Still working fine in Firefox, but quirky in IE6.
For a layout region with multiple tabs, whereby each tab hosts a grid, and all grids are loaded at the same time (i.e. only one grid visible at any one time) - upon switching to another tab, the grid rows are empty.
The grid rows reappears upon either a container resize or simply moving the mouse cursor over any of the invisible grid records.