FYI,
http://forum.frenchiphone.com/index.php/topic,3743.msg44610.html#msg44610
states that IPSF > 1.6 survives the update.
It also suggests that if you use an IPSF version prior to 1.6, you should reverginise it (that can be done in 1.0.2 only if I remember right) then update to fw 1.1.1 and apply IPSF 1.7-rc4.
My iPhone was unlocked with IPSF prior to fw 1.1.1 release (hence it's a very old version, pre 1.6 for sure), but I decided to update it to 1.1.2 without verginising and guess what ? It works. (the ipsf unlock survived the update).
Was not aware of SOME of that, however fantastic news.