Investigating/Troubleshooting I/O Spikes
I have experianced a similar issue a few of times in the last year, and each time it has been due to an external issue, normally anti-virus software with incorrectly configured exclusions. I would check this.

In the mean-time, the following query will let you examine the level of physical IOs comming from SQL Server:

SELECT a.io_stall, a.io_stall_read_ms, a.io_stall_write_ms, a.num_of_reads, 
a.num_of_writes, 
--a.sample_ms, a.num_of_bytes_read, a.num_of_bytes_written, a.io_stall_write_ms, 
( ( a.size_on_disk_bytes / 1024 ) / 1024.0 ) AS size_on_disk_mb, 
db_name(a.database_id) AS dbname, 
b.name, a.file_id, 
db_file_type = CASE 
                   WHEN a.file_id = 2 THEN 'Log' 
                   ELSE 'Data' 
                   END, 
UPPER(SUBSTRING(b.physical_name, 1, 2)) AS disk_location 
FROM sys.dm_io_virtual_file_stats (NULL, NULL) a 
JOIN sys.master_files b ON a.file_id = b.file_id 
AND a.database_id = b.database_id 
ORDER BY a.io_stall DESC 


Pay particular attention to IO_Stalls, Reads and Writes. This query cam from http://blogs.msdn.com/b/dpless/archive/2010/12/01/leveraging-sys-dm-io-virtual-file-stats.aspx