I just tested this on the PowerCMS demo in Windows 7 using Firefox 3.5.5 and was able to copy and paste the table - bit it isn't perfect.
There seem to be two issues at play here. First, you need to make sure you are selecting all the cells in the table, not the table itself. For some reason, the copying function is not enabling when selecting the table. So, click and drag from the bottom right cell to the top left cell and you should see a blue border around all cells.
Press Ctrl (Cmd) + C to copy this to the clipboard and Press Ctrl (Cmd) + V to paste.
The table should be pasted where your cursor was placed, but for some reason the borders are not displaying. I discovered if you switch to Source and then back to Design views, the borders appear. Also, if you press Ctrl + Z (Undo) and then Ctrl + Y (Redo), the table reinserts with borders visible as well.
