If Exists update else insert

[Juan E Suris]

I need to do something similar to replace, but can't figure out how to do it.

Here's my table:
user  varchar(15) PRI
cnt  int(11)

I want to increment cnt for user if it exists, else insert a row with cnt=1. I tried 
the following, but mysql complains that I can't use the same table in the update and 
select:

replace into table set user='someuser', cnt=(select cnt+1 from table where 
user='someuser')

Can this be done (in 1 query, of course)?

Thanks,
Juan E