Interesting. I'm definitely curious as to how this will all blend together.
One thing that has made all three of the big Memorial Day weekend events work so well is that each catered to a specific audience, so there is almost no fragmenting of the overall Bay area fanbase. BayCon was for the SciFi/Fantasy folks, Fanime for anime (duh

) and KublaCon for the gaming folks. There are those that migrate between them (I could be one if I tried hard enough, as I have friends and things I'd be happy to be involved with at all three events), but not a high amount, and most folks went to the event that held the most interest for them, usually from year to year.
A fourth event, even small, could put a ripple into the great "balance" of things, if you want to call it that. Not quite the ripple people thought would happen when say, AM2 was placed on the same weekend as AX, but depending on what the event is going to be like, it could be interesting to see how it places in with the others.
The convention that could be impacted hardest by this, in my opinion, is BayCon. Barring a steampunk-centric event, BayCon is going to have the potential for highest concentration of steampunk attendees, as it fits into the overall convention genre/theme.
As for reg delays, perhaps there is a deal being worked out on reg for the two events...say, if you buy a Fanime badge you get a discount on their badge, or something of that nature. Of course, that's assuming there will be anything like that because of the event "partnership" as it were.
All just inferencing at this point.