Hi, I tested DSM 3.0 in my simple testbed, with and without asyncron setting in RS407.
Unfortunable I didnt see any improvments but it might be my testbed.
Edit 2010-11-04
I did some more testing today with the same testbed (VM windows 2003 server,ESXI 3.5U5,non aligned discs and NBench) but with new resaults.
Bottom line is that
DSM 3.0 Rocks 
when it comes to NFS and ESXI 3.5.
I got 10 times better throughput on writes (no change in read) with DSM 3.0 and asyncron settings in NFS priviliges compared to older DSM versions.
The NFS performance was equal (sometimes better) to CIFS.
ISCSI still have low performance on writes in RS407.(no change in DSM 3.0)
Conclusion
The performance of ESXI 3.5 and RS407 with DSM 3.0 in NFS are equal to CIFS when asyncron NFS settings is used.
RS407 are a old NAS but performs well in my tests.
The new RS810 with a lot more processing power and memory will probably only improve the performance.
I expect ISCSI also to improve in RS810 vs RS407.
However I prefeer NFS when used with ESXI than ISCSI.(simplicity and "no" vm per volume restrictions compared to FC/ISCSI LUNS)
Now we feel that we can use synology as storage for ESXI.
The next challange is how to handle redundancy for the storage but thats an other topic.
I have no answer to why I didnt get this good resaults the first time I tested.
Edit 2010-12-01
After running more test (lager files and more threads) I have to change my statment above.
In DSM 3.0 the NFS have improved but its not as good as the CIFS performance, when I increase the load CIFS can perform 60% better in write than NFS and 10% better in Read, NFS is still useful but CIFS performs better.
When I increase the load on ISCSI the resource monitor in DSM 3.0 stops uppdating (Memory and/or CPU load?).
ISCSI in RS407 and ESXI 3.5U5 isnt usable.
But im not so sure that the RS810 will be our next low-end ESXI storage.
/R