What side dish should I have with saltimbocca?
I would venture to say there is no one specific traditional side dish.  If you go to Italy you aren't going to see saltimbocca with a side of anything on the menu.  You can pick you primi and contorni yourself.  I think s very simple mixture of roasted or grilled asparagus and/or squash would be very nice. 

If your wanting traditional and you are making a roman style saltimbooca (saltimbocca alla romana) You can try to pick a primi and contorni that would be common and traditional to that region.  For example you can do "bucatini all'amatriciana" as a primi. With a nice fennel and orange and/or artichoke salad as a side  And of course to the season it is being prepared.  To me that is really what is "traditional" Italian cooking.