My editing solution is Premiere Pro 1.5.1 with Aspect HD. For a project in august I need to mix 720/24p from a JVC HD100 and some 1080/50i from Sony HC1 and FX1. The main camera is the JVC so I want the 1080-clips to be downconverted.

It seem that just putting the clip on the timeline just crops and doesn't de-interlace the clip. I created a composition in After Effects and scaled the image down and it seem to match a bit better. Is that the best way to go? Any other alternative?

That works fine. You can scale to the timeline size and deinterlace within Premiere, although AE is more flexible for this. With Aspect HD 4.1 you could do the scaling de-interlacing and rate conversion (25/50i to 24p) as a single step during capture using the upgrade HDLink.

Neat! I have a license for the latest version but I haven't been able to change to it since my processor is too old for that version. So another reason to upgrade my computer then... :)