Here we are in December with no news. Sadly, I haven't found an alternative that gets so much right as Things does. But getting cloud wrong really sucks.
I don't use currently use Things, but I demoed it earlier this year. Using Dropbox, I was easily able to sync it across my three macs and it only took minutes to set up. Here's what I did to sync it across my Mac Pro, macbook pro, and iMac:
Step 1: Quit Dropbox on each computer.
Step 2. Intstall on the Mac Pro. This puts a file in the Applications Support folder called "Cultured Code". Library-->Application Support-->Cultured Code-->Things-->Database.xml
STep 3.
Make a symlink to the Cultured Code folder, then move the symlink to the local Dropbox folder on the Mac Pro. Dropbox-->Cultured Code symlink
Step 4. Repeat steps 2 and 3 for the macbook pro.
Step 5. Repeat steps 2 and 3 for the iMac.
Step 6. Restart Dropbox on the Mac Pro. Let it sync.
Step 7. Restart Dropbox on the Macbook Pro. Let it sync.
Step 8. Restart Dropbox on the iMac. Let it sync.
At this point, any changes you make on one computer will appear on the others **as long as you don't have Things open on more than one machine at a time**.
I currently do this for several applications that don't have cloud syncing (or didn't have it when I purchased them) including The Hit List, LittleSnapper, DVDPedia, & LicenseKeeper. Works without a problem for all of them. It should work for almost any database-type application.
Also, I don't have an iPhone, so I'm not sure how that changes the situation, but given that Dropbox has an iPhone app, I imagine it should work.
Hope this helps.
James