return to old domain name?

[lisag]

So we had old-domain which was well ranked in the SERPs by all major search engines, including Google. First page results for over 100 terms, PR of 6. About 67,000 pages indexed by Google on the site.

Two months ago, we changed domain names, leaving all pages and directory structures unchanged. Just the domain changed and we used a 301 in our .htaccess. Other search engines updated our links, no penalty. Google dropped the old domain name listings to supplemental results within 48 hours and the new domain name has been sandboxed for 2 months (except two pages, go figure).

Say we return to the old domain name given the impact this has had on our business. Any thoughts on how long it would take Google to return old-domain's SERPs pages back into the main index, if at all?