OK, I have come across an issue, that I had earlier today after "playing" around with SSPs (recreating an SSP), in that the Central Administration page would just come back with:

HTTP Error 503. The service is unavailable.

In the event log I was also seeing (amongst other issues):

Log Name: System
Source: Microsoft-Windows-WAS
Date: 7/1/2010 4:24:21 PM
Event ID: 5002
Task Category: None
Level: Error
Keywords: Classic
User: N/A
Computer: servername.domain.local
Description:
Application pool 'SharePoint Central Administration v3' is being automatically disabled due to a series of failures in the process(es) serving that application pool.

and

Log Name:      System
Source:        Microsoft-Windows-WAS
Date:          7/1/2010 4:24:21 PM
Event ID:      5139
Task Category: None
Level:         Warning
Keywords:      Classic
User:          N/A
Computer:      servername.domain.local
Description:
A listener channel for protocol 'http' in worker process '1920' serving application pool 'SharePoint Central Administration v3' reported a listener channel failure.  The data field contains the error number.

and

Log Name:      Application
Source:        VSS
Date:          7/1/2010 4:23:07 PM
Event ID:      8230
Task Category: None
Level:         Warning
Keywords:      Classic
User:          N/A
Computer:      servername.domain.local
Description:
Volume Shadow Copy Service error: Failed resolving account Administrator with status 1376. Check connection to domain controller and VssAccessControl registry key.
Operation:
   Initializing Writer
Context:
   Writer Class Id: {59b1f0cf-90ef-465f-9609-6ca8b2938366}
   Writer Name: IIS Metabase Writer
Error-specific details:
   Error: NetLocalGroupGetMemebers(Administrator), 0x80070560, The specified local group does not exist.

Earlier today I just un-installed and re-installed SharePoint. However, after it occurring again just now, I came across:

http://social.technet.microsoft.com/Forums/en-US/sharepointadmin/thread/11973578-e225-4d7f-b641-7645fe31cf39

Which states:

"It turns out that there is a "previously reported but undocumented" compatibility problem between MOSS 2007 and the IPv6 protocol that is causing this problem."

I therefore disabled IPv6 on the network adapters and carried out an IIS reset and indeed Central Administration came back. Other users have reported that all SharePoint sites come back with this error.