I've done a quick search of the rule book and found this interesting paragraph.
[urlnp=http://www.premierleague.com/staticFiles/2e/6d/0,,12306~93486,00.pdf]The Premier League Handbook[/urlnp]
Relegation
28. Subject to Rule B.29, the bottom 3 Clubs in the table at the end of the Season shall be relegated to
the Football League.
29. If any Club ceases during the Season to be a member of the League, the record of the League
Matches in which it has participated that Season shall be expunged from the table and the number
of Clubs to be relegated at the end of that Season shall be reduced so as to maintain at 20 (or, if
less, as near thereto as may be) the number of Clubs in membership of the League at the beginning
of the next Season.
30. If any Club ceases to be a member of the League other than by reason of relegation after the end of
the Season but before the Board has fixed the dates of League Matches for the next Season, the
Board may invite the relegated club which attained the highest position in the table referred to in
Rule B.20 at the end of the previous Season to rejoin the League.