Add a regular user account to your DB2 zone:

db2# mkdir -p /export/home/cmihai
db2# useradd -s /usr/bin/zsh -d /export/home/cmihai cmihai
db2# chown cmihai /export/home/cmihai
db2# passwd cmihai
New Password:
Re-enter new Password:
passwd: password successfully changed for cmihai
db2# su - cmihai
db2% cd /opt/IBM/db2/V9.1/bin

Check locale(1) and export LC_ALL=C if needed or db2 will complain:

db2% ./db2fs
couldn't set locale correctly

Make sure you read the install log in /tmp.

Here's a tip though: if you can, use the Graphical installer (ssh -X and run db2setup instead of db2_install).
All you need now is add various tuning, limitations and zfs quotas, etc.

If you need to start over, there's always ZFS snapshots or db2_deinstall.

Either way, if you need to create an instance:

# ./db2icrt -s wse -u db2fenc1 db2inst1
Sun Microsystems Inc. SunOS 5.11 snv_90 January 2008
Sun Microsystems Inc. SunOS 5.11 snv_90 January 2008
DBI1070I Program db2icrt completed successfully.

You can now use db2 to create a database and connect to it.

# db2
db2=> CREATE DATABASE test
db2 => CONNECT TO test
Database Connection Information

Database server = DB2/SUN64 9.1.1
SQL authorization ID = DB2INST1
Local database alias = TEST

db2 => CREATE TABLE clients (name char(25), surname char(50))
DB20000I The SQL command completed successfully.
db2 => LIST TABLES

Table/View Schema Type Creation time
------------------------------- --------------- ----- --------------------------
CLIENTS DB2INST1 T 2008-06-11-05.39.58.167896

1 record(s) selected.

db2 => INSERT INTO clients VALUES ('Some','Guy')
DB20000I The SQL command completed successfully.
db2 => SELECT * FROM clients

NAME SURNAME
------------------------- --------------------------------------------------
Some Guy

1 record(s) selected.