According to the documentation, when Etch is made stable, a new testing
will be made (in this case called Lenny), which is a direct clone of
Etch. Packages will migrate from Sid to Lenny at the same pace as usual
for the testing distro.
However, since Etch has been frozen, there are a lot of packages from
Sid that are ready for Testing, but can't go there. Lenny will catch
them when it is created, thus it is possible that the large number of
packages flowing in could cause some stability problems.
Does this clear things up?