failing to share internet access between two PCs installed with Debian 3.1

[Dominik Margraf <dominik.margraf@gmail.com>]

I have two computers, both initially installed with Debian 3.1 (Sarge)
RC3, The computer A has two ethernet ports and computer B has one
ethernet port.

eth0 of computer A is connected to the ADSL modem and eth1 of computer
B is connected to eth0 of computer B via a crossover cable.

For Computer A, I set

eth0:
DHCP assignment (in the 10.x.x.x range) between eth0 and ADSL modem

eth1:
IP:192.168.0.1
subnet: 255.255.255.0
broadcast: 192.168.0.255
gateway: not set

For computer B, I set

eth0
IP: 192.168.0.2
subnet: 255.255.255.0
gateway: 192.168.0.1

I also installed ipmasq on computer A.  Computer A was able to access
the internet.  Both Computer A and B could also ping each other
successfully and computer B was also able to share internet from
computer A

However after I reinstalled Computer A with the newly released debian
3.1r0a (with exactly the same settings described above) with ipmasq,
computer B could not obtain internet access any more despite computer
A still had internet access and computer B was also able to share CUPS
and NFS services from computer A successfully.  Also, computer B was
able to ping computer A and vice versa.

So what could go wrong here?  Is it because some problems with the
firewall and/or ipmasq?  How could I fix this problem?

Thanks

Dominik