Until now, someone told me once, that if your site is down or you do a hosting change, your traffic will go down alot in the next 2 months, specially in google. I allways think that was impossible, until now. One of my sites was down half day (gee!) and then i change it to another host. Well...CAOS is what i can say. The refresh of the dns was smooth, everything ok, but the traffic from google went down from 1.050 uniques day, to 66 uniques day, and after 1 week, still no growing at all. It's a big delay of 2 months, if my friend is right (i hope he's not wrong and the 2 months will turn in more months, geee!) So, if you don't know this yet, allways be aware of your site uptime and with hosting changes, it'll penalize you, at least for a time.
if you change hosts but keep the same domain name it should have no effect. Your site being down can have negative effects on your traffic. it usually goes back to normal after a few weeks.