I've got to start by saying, if you didn't update Swipe for another 6 months I'd still be happy.  The 3.5 update brought the only "must-have" feature on my request list, so everything else is just a bonus!  However, I won't hold back on ideas and challenges to test your Ninja Skills... 

If I may, please consider the following feedback on the new Tip feature:

The +/- buttons are dangerously close to the "Done" button.  We don't want someone to start ticking up that tip value then get sidetracked by accidentally hitting "Done."   A little more space between the Tip +/- and the "Done" button would be good... maybe make the "Done" button narrower, like a square?

I'd thought about switching the placement of the Tip  and the Total.  This would solve the "Done" conflict, however I don't think it's a good idea.  I think most people (right handers) use a right hand thumb or finger, and this would obscure the total as they're making the adjustment.  Plus, having the +/- near the "Done" really draws attention to it.

I do realize the +/- clickable area extends well to the left of the +/- buttons and over the % area... that's good.  But perhaps take it a step further to graphically indicate that, or even move things around a bit.

I think you could shorten the height of the signature panel a bit if you needed more space for the tip mechanism.  The extended width (obtained in the new landscape orientation) was crucially needed... the height I don't think is as essential.