There’s an free informative report at Vision Mobile that discusses use of the idle screen…
The main problem with this is that…
"Idle screen replacement requires integration of the AIS software with tens of relatively inaccessible APIs (application programming interfaces) which are only available to third parties subject to manufacturer approval. This implies that the AIS technology is mostly accessible to companies with strong relationships with handset and operating system vendors."
Replacing the idle screen under Symbian is only possible if you are a Symbian Partner.
The JAVA ME MIDP 2.0 specification doesn’t provide support to allow Java applications to use the idle screen. However, a few devices provide this extra non-standard functionality.
Using the idle screen creates some other considerations related to performance. Phone and battery performance can be affected when (especially network) active applications use the idle screen.
Active idle screens are likely to become more important as phones supporting widgets and MIDP 3.0 start to appear. (S60 will eventually allow links to widgets rather than embedding widgets themselves)