It looks like the 2nd (or 3rd) line in the SLN file has the following in the BE 2.0 SLN file:
# Visual Web Developer Express 2010
And has the following in the BE 2.5 SLN file:
# Visual Studio 2010
I don't think this information is critical or makes a difference in terms of whether VWD can open the SLN file. I'm guessing it's just a description that was added in there depending on who saved it last.
As far as I know, VWD can open and work with BE 2.5. Probably the more important factor is line 1 in the SLN file.
Microsoft Visual Studio Solution File, Format Version 11.00
I would imagine any version of VS 2010 can open a file that has this first line. This same first line is in the BE 2.0 and BE 2.5 SLN files.