It seems that checking if a parameter is null is not supported by SQL Mobile.
Also SQL Mobile doesn't support timestamps.
Thus the only two simple solutions I can think of were:
Having a column which is always set to newid() on any insert/update, just be sure that you always update it.
Having a column which is set to getdate() on any insert/update, again be sure to always update it. This would not be as effective in high concurrency where two updates could occur at the same time, but on a mobile device this is less likely.
In my case the use of the concurrency checks is just for sanity to make sure a different part of the progam doesn't try to commit a stale copy of a row (this shouldn't happen and is an error anyway).