A work-around that I've been using is this:
I start drawing, say, the line which I need to end exactly _on_ the skin line (not at its endpoint, though). With its start point clicked and the rubber band active, I hover the mouse over the wall's reference line, until the guide line shows indicating the wall's direction vector; then hover over to the wall's end (a bit more tricky if it's a long distance away) and hover the mouse over the relevant skin line's endpoint, then chose the new guide line with the // mark for the desired skin line; then, finally, move back to the original position and end my already-started line on the point where it's perpendicular to that last // guide line.
Like I said, it's quite involved and I would much prefer it if the boys and gals in Budapest did it for me (including this functionality in the next patch, or v13 at the latest).
On a side note, I wonder what the criteria are for including new features in new versions. If you check my other thread from last night, the one about Mouse Constraints not 'sticking' where I debate with Matthew Lohden, you'll read about my angst at their taking away a long-time functionality that was fundamental to drawing: when you define a direction vector, parallel or perpendicular to something, then you had it at your disposal until you defined a new one. Not anymore. You have to define your direction vector each and every time you need. Some programming hurdle, Matthew says.
MacBook Pro 17" 2.4GHz, MacOS X 10.5.6, ArchiCAD 11 & 12, WinXP, Vista (well, not really Vista...)