I'd make a little AppleScript that does this:
on GetDefaultWebBrowser() set _scpt to "export VERSIONER_PERL_PREFER_32_BIT=yes; " & "perl -MMac::InternetConfig -le " & "'print +(GetICHelper \"http\")'" return (do shell script _scpt) end GetDefaultWebBrowser do shell script "open -a \"" & GetDefaultWebBrowser() & "\""
Replace the URL (in quotes) with your homepage. The rest of the code serves to figure out what your default browser is, and make it the active application. That part lifted from a Daring Fireball article.
Save it as an app called Browser and put it somewhere. Once Spotlight indexes it, you can type Browser in and hit enter. This will open the URL in your default browser.
Open AppleScript Editor from your Applications/Utilities folder (if you have older than 10.6, it will be Script Editor in your Applications folder). Paste that line in. Go to File-> Save As. Set the File Format: menu to Application. Choose a location and save.