No, I meant that after 30 days, the photos in Photo Stream automatically delete from the server but not the device.
So, photos upload to the server, then any devices that connect to Wi-Fi within 30 days will receive the photos, devices that don't make the 30 day window won't be able to download photos. Devices that have successfully downloaded the photos will keep them stored past the 30 day window, if you delete a photo from one device then it will delete from all (Even after the 30 day window).
PhotoStream on a Mac downloads in the background just like Calendars/Reminders do, so no worries in having to open iPhoto within those 30 days, just need to make sure you connect to the Internet within 30 days.
Another thing is that PhotoStream doesn't back up to iCloud like the camera roll does, so if you restore a device then PhotoStream will only have photos that are <30 days old. And, one last thing, photos uploaded to PhotoStream are compressed (And I think resized) so if you want pics in the highest quality then it's best to transfer them from the camera roll, rather than using PhotoStream.