Many, many things. Most notably:

* Check CDROM insert-remove error of Matt Mueller
* Create wait-queues for bus locking instead of pure semaphores, to make
  it more interactive
* Check for block read/writes in smbus-on-i2c emulation code
* Fans give overflow if small values are written to them in /proc; other 
  features may too.
* At least the bit-lp and bit-velle modules do no detection on loading;
  ask Simon whether this is possible to add.
* kmalloc with GFP_KERNEL can cause a context switch! Check whether this is
  safe everywhere it is used.
* Support 10-bit addresses. At this moment, they are supported nowhere, except
  in Simon Vogl's i2c modules.
* Change the i2c modules to keep the namespace clean (EXPORT_SYMBOL does not
  work for us; many static declarations need to be added; better ask Simon
  Vogl first)
* Make it SMP-safe: there are no spinlocks yet. They are needed at many
  places, probably (everywhere where global vars are accessed). This must be
  done for the i2c modules, too.
* Better general locking, or at least a once-over to check no race-conditions
  remain.
* Make lm78.c detect 'double hits', (same chip connected to both SMBus and
  ISA).
* Better chip detection; insmod-time paramters to set addresses 
  (difficult, because of all i2c busses!).
* At least one case report that unused port addresses do not return 0xff
* Registrate which i2c addresses are used (a la ISA) for each adapter.
* Rename i2c-proc.c to i2c-user.c, and extend it with /dev entries;
  or, create a better i2c-dev.c, that understands SMBus commands.
* Write all other drivers
* How to make drivers detect thing *not*; for example, if a Winbond is 
  present, the LM75 driver should not try to access its simulated
  LM75 chips.
* Write a userland detection program for busses (clients are less important;
  but perhaps needed too).
* Some library routines are pretty inefficient right now
* Test SIS5595(/91/98) sensor support. Write generic i2c support. 
  Thomas Dickel <Thomas.Dickel@mni.fh-giessen.de> owns one SIS5595.
  The sensor data must be added to lib/chips
* Extend detect script to detect chips too.