WP 3.0.4 doesn't recognize need to update to 3.1

I have multiple disconnected WP sites. All but one picked up on the release of 3.1 and have been updated accordingly.

One of them (runs locally on a development machine, under XAMPP and Win7) simply refuses to do so.
I tried modifying the date value in the _site_transient_update_core record from wp_options: nothing.

I even deleted that record (via phpMyAdmin): nothing.
It joyfully recreates it, with a new date value and says I’m up to date.

Is there
A) a better way to ‘force a check for updates’ (other than modifying or clearing that record in the db) ?
B) a way to ‘force an update’ (other than doing it all manually) ?
C) any info on how WP checks for updates? I’m wondering if the network this machine is on (at work) is blocking some traffic and therefore WP can’t get to the server showing the update is available (and then joyfully reports that as “there are noi updates for you”) ?

Thanks in advance,
FR

PS: Yes, I disabled all the plugins, just in case