Is there a web-based iCloud interface for editing items?
Since your iPhone let you sign up for an iCloud account, you have full access to the web services provided by iCloud.com, which include:


Contacts
Calendars
Mail
Find My iPhone


You are only limited by your computer, but still have access to the web tools. 

There may be (unsupported) ways to get some of these applications on your 10.5 Mac to sync, especially with Mail. Most of iCloud uses standard networking protocols, and therefore older programs may be able to use them. One may want to tread lightly though, since this is unsupported, and may cause issues/corruption in other parts that are supported (ie, you wouldn't want to edit your contacts in a way on 10.5 even if you could, since it could break them on your iPhone).