<p>How do you insert/update a column through Linq To SQL and Linq To SQL use the default values?  In particular I'm concerned with a timestamp field.</p>

<p>I've tried setting that column to readonly and autogenerated, so it stopped trying to put in DateTime.MinValue, but it doesn't seem to be updating on updates.</p>

The database default value would only insert the value on creating the row.  It would do nothing for update, unless you want to add a triggger.
Alternately, you can add a partial method to your DataContext class.  Add a new file you your project:
```
```
public partial class YourDatabaseDataContext
{
    partial void InsertYourTable(YourTable instance)
    {
        instance.LastUpdateTime = DateTime.Now;

        this.ExecuteDynamicInsert(instance);
    }

    partial void UpdateYourTable(YourTable instance)
    {
        instance.LastUpdateTime = DateTime.Now;

        this.ExecuteDynamicUpdate(instance);
    }
```
```