From what I can see CLOB / CLOB support varies for these databases and 
drivers:

1) MySQL 4.0.16 using Connector 3.0.8 driver: Excellent (1,000,000 
char clob works)
2) Oracle 9.2.0.2 using Oracle thin driver 9.2.0.3: Good. Works for 
4000 char clobs, but I get errors trying to deal with anything 
bigger.. possibly a sqlplus option that needs to be set.
3) Microsoft SQL Server 7.0 using jTDS 0.5 / 0.6: Clob support is 
expected to be fully built into 0.6 final version, although 0.5 has 
some support (I haven't yet tested this with Hibernate though)
4) Sybase ASE 11.9.2 using Sybase jConnect 5.5 driver: Clob support is 
missing.  It seems the driver uses a proprietary API to implement CLOB 
("text" in Sybase terminology) support.