Barry McMillan of the American Enterprise Institute has looked at this trend and notes that many of the now-Democratic seats are in districts that had not only been held by GOP legislators but carried by President Bush in 2004.
The link for the report isn't up yet at AEI's political corner page, but should be soon. Here's the relevant info, referring to special elections held from December 2004 through May 2006:
"Democrats have picked up 18 formerly Republican seats, 15 of which were in districts won by President Bush. By contrast, Republicans have picked up only seven seats, only two of which were in districts Senator Kerry won. Many of the Democratic gains have been in suburban districts."