As long as the site and URL structure remain unchanged, then a server migration shouldn't make any impact in the rankings.
That being said, there are a few relatively uncommon things that could potentially cause some issues -
1. If you are going to be moving from a PHP server to an IIS server and you need to change your URLs to .asp or something like that, then you make take a temporary hit while the search engines crawl and index the new versions.
2. If the new server is slower or is prone to frequent downtime, then yes, it could negatively affect your rankings.
3. If you are moving to a new shared server, make sure the rest of the sites on that server are all legitimate sites that haven't been blacklisted by the search engines for any reason. For example, if you move your site to a server that has 100 Russian sites or Nigerian spammer sites all on the same IP, then it's possible your site could be negatively impacted.
Again, these are all relatively uncommon scenarios as long as you are simply moving your site over to another server while keeping everything else the same, so you shouldn't notice any difference in your rankings.