Worthwhile optimisation of position()?

[Christopher Kings-Lynne <chris(dot)kings-lynne(at)calorieking(dot)com>]

Is it worth allowing this:

select count(*) from users_users where position('ch' in username) = 0;

To be able to use an index, like:

select count(*) from users_users where username like 'ch%';

At the moment the position() syntax will do a seqscan, but the like 
syntax will use an index.

Chris