Seb,
as per georg reply the big blind is still 2000. If the BB seat was all-in for 1800 (1000/2000 blinds) (200 ante 1600 main pot) each respective player would still have to either call 2000 (the BB forced bet) or raise a minimum of 2000 to play with the extra being established into a side pot:
example 1:sb2,000 bb4,000 a500 10 players
bb seat is all in with 3,200 (so 2,700 + a500)
utg (4,000 to play) calls 4,000
cut-off raises (min must be 8,000) to 11,000
all others fold
utg calls 7,000
main pot would be 3x2,700 + sb2,000 = 10,100 + a5,000 = 15,100
side pot would be 2x1,300 = 2,600
example 2:blinds 100/200 10 players
3 players (sb bb & button) see a flop with the pot at 600
sb checks, bb bets 300, button moves all in for 480, sb announces raise (min 300 to at least 780)
example 3:3 players blinds 100/200
a checks, b pushes all-in for 150, c calls
player a can only call or fold this time as their has not been a full bet made by player b and he has given up his oppertunity to raise by checking the round. If player C raised he would be able raise again
In all events the big blind stands as the pre-requisite for playing your cards. Here's a quick video of Steve Frezer, one of the WSOP TD's talking about 5000/10000 level when the BB has only 5000. Bear in mind he is talking about pot-limit though which is slightly different as the pot size determines betting limits.
http://www.youtube.com/watch?v=GmsDtNwp89gHopefully you will understand the under raise and under forced bet issues better. If you do need further clarification please do not hesitate to post up.