If you have to switch to another activity, tap "start" on the new activity. The old item is paused and can be reactivated at any time.
In case you have to report times spent on subtasks, you can very easily create a hierarchically ordered set of activity items. A started sub item automatically updates the total time of the parent item(s).
If you put a parent item on pause (e.g. via a switch to another item at the same level) an active sub item is automatically paused, too. This sub item is then displayed with status "yellow" instead of "red" because it wasn't stopped explicitly but indirectly because of the status change of its parent item. If you later restart the parent item, the previously active sub item timer is automatically restarted, too.
iTimes TimeTracker can be closed to use other iPhone/iPodTouch functions and applications while a timer is active. It will then show a red badge on the application's icon to remind you that a timer is still running.
iTimes TimeTracker was developed with business use in mind but it's great tool for private activities, too. E.g., for sports activities like jogging or inlining it's great to record times for predefined route sections and the total route time.
If you create day based group items you can always compare your new times with yesterdays results.
Link to iTimes TimeTracker: iTimes TimeTracker on Apples’ iPhone App Store (local iTunes installation needed)