Does anyone know of a timeline on a permanent fix yet?
It doesn't look like there's going to be a permanent fix, at least not from VeriSign's end. Below is the text of an e-mail I received this morning from the VeriSign rep who's been addressing my trouble ticket. The big problem appears to be that the Payflow Link server will no longer follow redirects. My stores, and I presume all of those using the same Payflow Link module for osCommerce, have Payflow Link silent POSTing to "verisignreturn.php", which then redirects to checkout_process.php. Someone in the community is apparently going to have to rewrite the Payflow Link module so that it doesn't incorporate a redirect. I'll probably take a shot at it some time in the next few days if no one else beats me to it.
There may be other minor issues also playing into this problem, and so here's the text of the e-mail I got from my VeriSign rep, which (hopefully) contains a complete list of differences between new server and old.
I understand you've tried using our new Payflow Link service @
https://payments.verisign.com/payflowlink w/ osCommerce, and were having
difficulties with the Silent POST under the new service.
I also understand you were given a different URL that uses the old service
-- while that will work for the time being, please be aware that it is in
temporary operation only, and it *will* go away within the next two months.
We need to get you up and running on the new URL/service.
That said, here's a couple of significant differences to the new service:
1) HTTPS Silent POSTs *will* work, but you need to get rid of the port
specification in your URL. I.e. say 'https://me.com/silent.php' instead of
'https://me.com:443/silent.php'.
2) Silent POST scripts that issue redirects of any kind will *NOT* work any
longer -- no <META> tags, no 30x Status: headers, none of that. Proxied URLs
should work, but not redirects. Take a close look at your specific script to
see if it does anything in this regard.
3) The format of the new Silent POST is ever so slightly different in the
following ways:
a) it's still URL encoded, but spaces are now encoded as '+' rather
than '%20' -- no PHP script should have any problem with this.

there is one extra name-value pair coming back now, 'PONUM',
which unfortunately slipped through our documentation.
3b) *might* be a problem -- I have already seen at least one very fragile
Silent POST script that died when presented with a new name-value pair.
You might for now wish to create a new, testing-only Payflow Link account --
that way you can set up a second set or URLs to test on, leaving your public
site untouched until you're ready to switch.