For the time being I would recommend all of those trying to sync google calendar with N900 calendar to use Nuevasync (www.nuevasync.com). Get a free account there and give Nuevasync permission to stay in sync with your google account. Then, the website will tell you to set up a mail for exchange profile using Nuevasync's server settigs. Your N900 will sync just perfectly with Nuevasync, which in turn is in perfect sync with google.

On the website you can choose which google services you want to sync. Options for a free account consist of calendar, tasks, contacts (works extremely well, got 150+ contacts in sync without missing info / misplaced info / etc). Push e-mail is only available for premium (paid) accounts but that's not a big problem since the N900's built-in mail application works quite well with google mail.

So basically, Nuevasync is a perfect solution until Nokia finally manages to sync the N900 properly with google services, and maybe even after that. Spread the word.