I'm guessing that there is no real problem and that the reason the beta fails to install is that it checks for version 10.5.6, which with 10.5.7 it obviously doesn't find, so installation cannot proceed.
Apple are probably putting it in the seed notes so that people know that it won't work. The next Safari beta will fix this.. if I'm right, that is.
This seems extremely likely to me. If you read the seed notes you can see that stuff unrelated to Safari is getting fixed in each build.