First of all many thanks for the instant replies, but I'm afraid that's 
not all of the solution ... if I do not use the le0 interface but the 
qe1 instead (rename /etc/ifconfig.le0 to ifconfig.qe1) and reboot I do 
not get a route as well. Ipforwarding is set in sysctl.conf. Here an 
excerpt of dmesg:

bpp0 at sbus0 slot 4 offset 0xc800000 level 2 (ipl 3): dma rev 2
ledma0 at sbus0 slot 4 offset 0x8400010: dma rev 2
le0 at ledma0 slot 4 offset 0x8c00000 level 6: address 
08:00:20:19:20:2d
le0: 8 receive buffers, 2 transmit buffers
qec0 at sbus0 slot 1 offset 0x20000 level 4 (ipl 7): 128K memory
qe0 at qec0 slot 0 offset 0x0 rev 1 address 08:00:20:19:20:2d
qe1 at qec0 slot 1 offset 0x0 rev 1 address 08:00:20:19:20:2d
qe2 at qec0 slot 2 offset 0x0 rev 1 address 08:00:20:19:20:2d
qe3 at qec0 slot 3 offset 0x0 rev 1 address 08:00:20:19:20:2d
SUNW,DBRIe at sbus0 slot 2 offset 0x10000 level 9 not configured

.. and from ifconfig -a:

le0: flags=8822<BROADCAST,NOTRAILERS,SIMPLEX,MULTICAST> mtu 1500
	address: 08:00:20:19:20:2d
	media: Ethernet autoselect
qe0: flags=8863<UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX,MULTICAST> mtu 
1500
	address: 08:00:20:19:20:2d
	media: Ethernet autoselect (10baseT)
	status: active
	inet 192.168.0.1 netmask 0xffffff00 broadcast 192.168.0.255
	inet6 fe80::a00:20ff:fe19:202d%qe0 prefixlen 64 scopeid 0x2
qe1: flags=8863<UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX,MULTICAST> mtu 
1500
	address: 08:00:20:19:20:2d
	media: Ethernet autoselect (10baseT)
	status: active
	inet 10.0.0.3 netmask 0xff000000 broadcast 10.255.255.255
	inet6 fe80::a00:20ff:fe19:202d%qe1 prefixlen 64 scopeid 0x3
qe2: flags=8822<BROADCAST,NOTRAILERS,SIMPLEX,MULTICAST> mtu 1500
	address: 08:00:20:19:20:2d
	media: Ethernet autoselect (10base5)
	status: no carrier

.. finally from route show:

Internet:
Destination       Gateway            Flags 
10.0.0.0          link#3             U      
10.0.1.50         00:02:44:63:cb:d8  UH     
loopback          127.0.0.1          UGR    
localhost         127.0.0.1          UH     
192.168.0.0       link#2             U      

With this configuration not using the le0 interface I  get "no route to 
host" for every ping. Only networks "seen" by le0 appear to be pingable 
..

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