The roads I take...

Yes, what the recent Calendar blog post says is true: I did enable the Lightning Calendar to be built with SeaMonkey.

But be warned: NO, it DOESN'T WORK - currently, at least.

Recently, when the way of integrating Lightning with Thunderbird has been changed, the integration with SeaMonkey broke heavily and we still need to work on fixing that.

There are a few other things to be aware of as well:

• Lighting does NOT work with SeaMonkey 1.x and never will. The Calendar team is very small and working on huge tasks (pun somewhat intended), so they needed to stop supporting the old, obsolete xpfe toolkit we are using in SeaMonkey 1.x - there is a privately maintained port of the old calendar extension that is said to work with SeaMonkey 1.x in the meantime.
  The work for making Lightning work with SeaMonkey does only affect the development "trunk" and SeaMonkey 2.
• Lightning is not at the moment planning to do any official release with SeaMonkey support at this point. Currently, Sunbird and Lightning releases are still done from and for the 1.8 branch to support Thunderbird 2.0 as well as possible, trunk support is some kind of a stepchild to the calendar team until Thunderbird 3 gets worth supporting.
• Someone needs to work on SeaMonkey-Lightning integration. While some members of the calendar team are eager to see additional users and testers due to SeaMonkey support, it's clear that their team is unable to work on Lightning's SeaMonkey integration themselves. We need to find someone from the SeaMonkey side, ideally someone strongly interested in calendaring support for SeaMonkey, to make this working and keep it going. I'm sure they'll accept patches though, esp. if someone comes up who wants to maintain that integration permanently. There's a special Bugzilla component in the Calendar product that deals with this integration now.
• The calendar team doesn't accept SeaMonkey-only bugs. If you encounter a bug with Lightning withing SeaMonkey, go and test if you see this in Sunbird and Thunderbird+Lightning before reporting it in Bugzilla. The calendar team can't work on SeaMonkey-specific issues at the moment and won't care too much for those. If the bug is about how Lightning integrates with SeaMonkey, feel free to report it in the specific component we now have for that, but don't expect someone to work on it. As I said, we are still looking for someone to work on this.
• We have no firm plans or decisions to ship Lightning with SeaMonkey yet. Even this is an interesting option for the future, we first need to see how well it works and if we can find someone working on the integration before seriously talking about this.

All that said, we are happy we can cooperate with the calendar project to get some kind of calendaring support for future SeaMonkey versions, as this is a frequently requested feature and an interesting prospect for addition to our suite. Still, there's lots of work to do and lots of experience to gather - we'll see how this all works out.

Thanks to the calendar team for enabling us to work with them on this.
And if you are interested in good integration of Lightning with SeaMonkey and willing to work on patches and maintaining them - we are looking for you!