I’ve tried to reproduce the problem on a fresh virtual environment and I had the same behavior. I think this is a major issue that needs to be fixed ASAP.
Here is the configuration:
pfSense 1.2.3-RC3 built on Tue Oct 13 03:53:26 UTC 2009
LAN interface (de0)
IP address 192.168.1.1
Subnet mask 255.255.255.0
WAN interface (de1)
IP address 10.1.1.223
Subnet mask 255.255.0.0
Gateway 10.1.2.1
OPT1 interface (de2)
IP address 10.0.0.201
Subnet mask 255.255.255.0
Gateway 10.0.0.1
All the rules and NAT settings of the firewall are the default of the initial setup wizard, except the rule on the LAN where I’m telling that everything from the LAN to be routed through 10.0.0.1 (OPT1 and NOT WAN).

Not so important but both WAN and OPT1 networks are connected to the internet through 2 separate external firewalls.
Client configuration – Windows XP
LAN adapter
IP 192.168.1.100
Subnet mask 255.255.255.0
Gateway 192.168.1.1
Some traceroute examples form the client computer: (explains everything)
C:\>tracert 1.1.1.223
Tracing route to 1.1.1.223 over a maximum of 30 hops
1 2 ms 1 ms 1 ms 10.0.0.1
2 20 ms 19 ms 19 ms 222.145.225.175
^C
C:\>tracert 1.1.1.224
Tracing route to 1.1.1.224 over a maximum of 30 hops
1 <1 ms <1 ms <1 ms 192.168.1.1
2 16 ms 19 ms 19 ms 85.139.62.193
^C
C:\>tracert 1.1.1.239
Tracing route to 1.1.1.239 over a maximum of 30 hops
1 <1 ms <1 ms <1 ms 192.168.1.1
2 17 ms 19 ms 19 ms 85.139.62.193
^C
C:\>tracert 1.1.1.240
Tracing route to 1.1.1.240 over a maximum of 30 hops
1 2 ms 1 ms 1 ms 10.0.0.1
2 29 ms 17 ms 19 ms 222.145.225.175
^C
C:\>tracert 54.54.54.223
Tracing route to 54.54.54.223 over a maximum of 30 hops
1 2 ms 1 ms 1 ms 10.0.0.1
2 * 29 ms 19 ms 222.145.225.175
^C
C:\>tracert 54.54.54.224
Tracing route to 54.54.54.224 over a maximum of 30 hops
1 <1 ms <1 ms <1 ms 192.168.1.1
2 14 ms 19 ms 20 ms 85.139.62.193
^C
C:\>tracert 154.154.154.223
Tracing route to 154.154.154.223 over a maximum of 30 hops
1 2 ms 1 ms 1 ms 10.0.0.1
^C
C:\>tracert 154.154.154.224
Tracing route to 154.154.154.224 over a maximum of 30 hops
1 <1 ms <1 ms <1 ms 192.168.1.1
^C
C:\>tracert 154.154.154.240
Tracing route to 154.154.154.240 over a maximum of 30 hops
1 2 ms 1 ms 1 ms 10.0.0.1
^C
Conclusion: If the IP address ends with a number between 224 and 240, the LAN rule to route the packet through OPT1 interface is ignored.