in-memory sorting

I've got this explain: http://explain.depesz.com/s/Xh9

And these settings:
default_statistics_target = 50 # pgtune wizard 2010-08-17
maintenance_work_mem = 1GB # pgtune wizard 2010-08-17
constraint_exclusion = on # pgtune wizard 2010-08-17
checkpoint_completion_target = 0.9 # pgtune wizard 2010-08-17
effective_cache_size = 36GB # sam
work_mem = 288MB # pgtune wizard 2010-08-17
wal_buffers = 8MB # pgtune wizard 2010-08-17
#checkpoint_segments = 16 # pgtune wizard 2010-08-17
checkpoint_segments = 30 # sam
shared_buffers = 11GB # pgtune wizard 2010-08-17
max_connections = 80 # pgtune wizard 2010-08-17
cpu_tuple_cost = 0.0030                 # sam
cpu_index_tuple_cost = 0.0010           # sam
cpu_operator_cost = 0.0005              # sam
#random_page_cost = 2.0                 # sam

I'm not understanding why it is sorting on disk if it would fit within
a work_mem segment - by a fairly wide margin.  Is there something else
I can do to get that sort to happen in memory?