Scalability with large numbers of tables

Dear Postgresql experts,

I have a single database with one schema per user.  Each user has a
handful of tables, but there are lots of users, so in total the database
has thousands of tables.

I'm a bit concerned about scalability as this continues to grow.  For
example I find that tab-completion in psql is now unusably slow; if
there is anything more important where the algorithmic complexity is the
same then it will be causing a problem.  There are 42,000 files in the
database directory.  This is enough that, with a "traditional" unix
filesystem like ext2/3, kernel operations on directories take a
significant time.  (In other applications I've generally used a guide of
100-1000 files per directory before adding extra layers, but I don't
know how valid this is.)

I'm interested to know if anyone has any experiences to share with
similar large numbers of tables.  Should I worry about it?  I don't want
to wait until something breaks badly if I need architectural changes.
Presumably tablespaces could be used to avoid the
too-many-files-per-directory issue, though I've not moved to 8.0 yet.

Thanks

Phil.