Use the "cut sheetmetal" command in the tools menu; which is essentially boolean for sheetmetal.
How to use:
1) Create a part out of regular shapes that will act as your cutting tool. This MUST be a seperate part, not a shape; so be careful when you drop it into the scene so that it does not drop as a shape.
2) Position your cutting tool through the sheetmetal
3) Select the sheetmetal that you desire to cut at the part level, the shift+select the cutting tool (both should be selected in blue)
4) Pull down the tools menu and choose "Cut Sheetmetal Part". Nothing will seem to happen when you do this.
5) Suppress the cutting tool, notice the hole.