Brainstorm

when adding a calendar event, there should be a field where one could define the online status that should be activated during the event. this could apply for both phone profile (ringing tone, volume, sms alerts, vibration) as well as online statuses for IM & facebook.

This is basically almost a combination of solution 1 and 3. There would be an extra field in Calendar where one could choose the desired Profile (since, if I have understood correctly, the N900 can have multiple Calendars, reoccurring events could be easily set in the Private Calendar that does not sync). In Profiles you would have an extra setting for IM status. Also there should be no limit on how many Profiles a user could add in addition to the default (General and Silent) ones.

Each profile should have the option of setting the IM status when the profile is activated or (by default) to use the existing one.

Each Calendar entry should be able to set the profile (with the default being use existing) and to set the IM status (with the defult again being to use existing).  This would allow the user to tell the Calendar event to override the profile's IM status or to set status by event, even if they don't want to associate IM status with profiles.  The status should revert to the previously existing status at the end of the Calendar event's period, unless the user has changed the stus in the interim.

The Clock app's alarms should have the same options as Calendar entries to set profile and IM status.

a fully integrated profiles-IM-calendar-system-controler setup based on native environment (no additional programs!)

• settings to choose in your profiles what you like to have on/offline, ICQ online MSN offline WiFi online GSM offline SIP1 online Skype offline (or even change accounts for IM and voip if double entries for one network arent allowed and you got separates for work and private)etc.

• settings for calendar entries to choose which profile BUT also an override for your status and system settings, your profile pops up and you may change what you like but changes wont last longer as your calendar entry does(you are in a meeting and want everything muted instead of turned off, radiobuttons for all posibilities)
• a weekly-based scheduler attached to your profiler (from work to home to sleep and the way back), gets override from calendar entries and if it changes after the scheduled event to your weekly preference
• a vacation setup, mark your business accounts and prevent them to go online, office weekly-schedule entries dont change your profiles

ContextKit could provide calendar integration, but also a lot more (see http://maemo.gitorious.org/maemo-af/contextkit/blobs/master/spec/core.context for ideas).   This is such an obvious use case for it that it's probably planned for Harmattan.