SVN+SSH tunneling problem

[I decided to repost this from thread "SVN+SSH access not advisable"
since it is not fully related to that discussion.]

---
I created a svn-user named svnuser. In his homedir I created on the server 
a repo only accessible to him (/home/svnuser/svntest).
His .ssh/autorized_keys looks like this:
---
from="myclient.net", \
command="svnserve -t --tunnel-user=USER", \
no-port-forwarding, \
no-agent-forwarding,no-X11-forwarding,no-pty \
ssh-rsa ...KEY... COMMENT
---
That the key ...KEY... is correctly assigned to the user svnuser I could 
check when I did a checkout on my client via path
	svn+ssh://svnuser@myserver.net/home/svnuser/svntest/trunk
Working on this copy is possible.
Drawback though: the author still shows up as svnuser, although option 
--tunnel-user was set to USER!
I suspect that the command specified is not called correctly, since even 
if I *misspell* svnserve the svn access doesn't fail...
What goes wrong here?
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