This is known. The shift is not constant across the Russia, so we can't add a fix on the server side. Users should use closemost GPS traces to tie imagery precisely before any editing session.

Sorry I was too optimistic about its coverage. Asia is covered less than 10%. Here is coverage picture:

http://img146.imageshack.us/img146/3251/irsmap.jpg

Scanex says that in future there would be updates.

JohnSmith. haven't RTFS yet, but I suppose slippymap plugin expects merkartor projection on sphere. Scanex uses projection on WGS84 ellipsoid. So, calculating Y<->Latitude is different, and thus just adding an URL won't work. Probably slippymap plugin can be patched to gain support of non-sphere projections.

greencaps, you just need to insert this link in josm configuration. F12 -> choose WMS tab on the left, click "Add" button.