I setup my host's port 1521 to forward into the vm to simplify the connect. Here's the command to issue on the host in my case my mac.
Kris-Rices-MacBook-Pro:~ klrice$ VBoxManage modifyvm "Oracle Developer Days" --natpf1 "tns,tcp,127.0.0.1,1521,,1521"
Now anything going to 1521 on the host will be forwarded into the guest. Here's sqlplus working on the host machine.
Kris-Rices-MacBook-Pro:~ klrice$ sqlplus system/oracle
SQL*Plus: Release 10.2.0.4.0 - Production on Fri Sep 24 11:29:03 2010
Copyright (c) 1982, 2007, Oracle. All Rights Reserved.
Oracle Database 11g Enterprise Edition Release 188.8.131.52.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL> select 1 from dual;