It may have escaped your attention but support from RIM for version 4.0.x will be stopped by RIM from 31st January 08. See this for more information BlackBerry
Implications - BES will continue to run but if you run into issues you might well be told "upgrade" to solve the problem. There will be no further service packs, MR releases or similar released on 4.0
Consequences - trying to explain to CxO that ahem his shiny BB won't work anymore, and erm no you can't fix it quickly.
What to do - Get some budget/resource/timespace to focus on upgrading to 4.1.4. Read the RIM documentation, and then do some searching on this forum as it's been covered many times. Upgrade to 4.1 was a chargeable one from RIM so lean on carrier rep to get a freebie upgrade.
Timescale: Hard to say for sure but upgrading from 4.0 to 4.1 should take no more than a coupe of hours for a single BES (& DB Instance)
Precautions: Backup, Obtain original installation media, take manual records (or put on another server) details of SRP ID & Auth Code, and CALS
User Implications: None for 4.0 to 4.1 but their user experience will improve especially if using a 4.2 OS device, as will permite enhanced PDF, PPT and audio file opening
IT Implications: Different look and feel of the front end, different ways of achieving same outcome. More IT policies, Group based administration. Role based BES usage - permit the helpdesk guy to do basic checks/fixed without givine them full BES access.
Mods - can you sticky this for 3 months - there's lots of people posting on here that seem quite new and a lot of them don't seem to have heard of the EOL in 3 months.