Sybase and MS-SQL have a datatype called "timestamp" (which isn't time related at all) that can be used for this. It is an 8 byte binary column that gets updated each time the row is updated, so you can easily check if the data that the user has been editing was changed when applying the update. This is essentially the same as keeping track of changes with a counter, but happens automatically (and the timestamp column can't be modified manually).
The alternative is as others have suggested to implement your own locking mechanism via a secondary table. Be sure to include a time when the lock was acquired so that you can force the release of stale locks.
Michael
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