Execution Plan Mystery

I was hoping someone could shed some light on wierd situation i'm experiencing. I have the following query:

select count(*) LeadCount
from auto_leads al
where received > dbo.GetDay(GetDate())

dbo.GetDay simply returns a smalldatetime value of today's date.

Now I recently got thrown into a data mess and for some reason this query takes 8 seconds to run. Now the first thing I did was update the stats on the Received column of this auto_leads table. I re-run the query and I'm still getting 8 seconds. I look at the execution plan I can make figure out why this is happening.

I then change the above query so the filter received > dbo.GetDay(GetDate()) is now just received > '5/31/2006' and the query comes back immediately. This doesn't make sense to me because the GetDay function is really simple and comes back immediately. I then try the following query to confirm it isn't a problem with the GetDay function:

declare @Today DateTime

set @Today = dbo.getday(GetDate())

select count(*) leads
from auto_leads al
join type_lead_status tls on (tls.type_lead_status_id = al.type_lead_status_id)
where received > @Today

Sure enough, the query came back immediately. Next thing to go through my mind is that the query execution plan has been cached by SQL Server using the execution plan from before I updated the stats on the received column. So I executed sp_recompile 'auto_leads' and tryed the original query again. Still taking 8-10 seconds to come back.

So my question, is why when I remove the GetDay function call in my query filter is the query slow, as opposed to me just passing a variable into the query? Thanks!

- James