I didn't wade through this but you probably should:

In Revenue Rulings 99-5 and 99-6 the Internal Revenue Service ("IRS") discusses the federal income tax consequences of a single-member, disregarded LLC acquiring a second member and of a two-member LLC becoming a single-member LLC. In addition, in Notice 99-6 the IRS provides two temporary safe harbors regarding employment tax reporting and payment compliance by certain disregarded entities, including single-member LLCs, and invites comment on various aspects of application of the federal employment tax regime to these entities.

http://pmstax.com/part/disllc9901.shtml