Eventually this will be implemented in the Sd5 release but not in SD5.1. The first migration will be SD45SP17--->SD5.10 (aroun the summer) Next will most likely be SD4.5SP18--->SD5.20 (no scheduled timeframe yet).
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I take the same view as Gyula at the moment. 4.5 with the latest SPs is quite stable and can be tailored quite well however SLM is disapointing and not being able to use dynamic forms is a pain. OVSD 5.0 has some excellent features but appears to be buggy at the moment.
Service Desk 4.5 is quite mature and though SD 5 has some really needed features I think we can stay to SD 4.5 for the time being.
We need dynamic forms and unlimited custom fields more than SD 5.1 and furthermore, upgrading to 5.1 won't be a weekend's job.
So we decided to wait and see what will going to happen with 5.1 since we believe that the migration won't be painless and we'd rather wait others to find the problems than finding them by ourselves :)
According to plan (though we've waiting 2 years for SD5) 5.1 will be released around summer. So if you have the patience to go to 5.x around 2007 maybe it would be better to wait 5.2 that will incorporate all 4.5 features.