Anyone who has done a cross forest public folder migration will almost certainly be reliving their nightmares about it simply from reading the title.
I was just the same.
Extract the content to a PST file, either manually (selecting about 1000 items at a go) or by using a rule, move the PST file to a machine in the new forest, then import.
Slow, mind-numbing dull and therefore not the most fun part of a migration and always the bit that I don't look forward to.
However a recent migration was done almost completely hands free. I moved almost 9gb of data in an afternoon, while I went to the cinema.
To do this, I took advantage of the new feature of Outlook 2010 that allows it to connect to two different Exchange organisations at the same time.
This allowed me to create a rule to move the content between the two public folders. Once the rule was set, I left it to get on with it. The speed wasn't great, but compared to moving it manually, it was a considerable time saver. After returning from cinema I was able to do more of the migration work, while I waited for the rule to finish.
Furthermore, by using multiple machines, I could move lots of large public folders at once. Once the process was completed, the rule was discarded.
Even before moving the data, when creating the new folders it was easy to setup the permissions as I could compare them side by side.
Where an item was corrupt and couldn't be moved, or the few items that didn't match the rule, I simply moved those items manually or deleted them. In most folders this was only a few hundred items at most.
You still can't copy and paste large numbers of items, as the problem with trying to copy/cut and paste more than about 1500 items is still in Outlook, but a rule effectively moves each item individually, so that isn't a problem.
For the folders with small number of items, a straight copy and paste works well.
I used the same procedure to move a stubborn mailbox which wouldn't move on the regular cross forest move mailbox procedure. Much faster than exporting the mailboxes out to PST file and then importing them. It also allowed me to identify the corrupt items and deal with them.
Even if you aren't deploying Outlook 2010 for your migration, it is worth downloading the trial for this feature alone. Then once the migration is complete, discard the trial software.