Depends on how long...One of my sites was down this morning for about 8 hours...no change in any rankings.
Normally, if it's for a short period, nothing happens. I had a hosting account with a *bad* ISP who suspended me after I had paid my bill, and it took about 12 hours to get set up with a different ISP and for the DNS changes to propagate - no change in the ranking, but many concerned emails from frequent visitors.
I was once down for 3 days due to an FBI investigation on a shared server (yeah right, I think the host just fried the server and had no replacement) It didn't effect me but I think I was actually lucky in that case
Downtime period may concur with the SE bot crawling of your site. So it could cause to no or old information about your site pages in some search engine(s) before site is crawled again.
thanks for the replies. my client's site have been down for about 4 days. problem with the hosting also