My wife's bike is creaking while standing. I replaced the bottom bracket and lubed the crankset, pedals,.... Still creaking, so I am turning to the headset next. I haven't pulled it apart yet, but what I have read on the trek site it sounds like somewhere between a press-fit and the current integrated bearings (headset w/semi-cartridge bearings)

Any insight on are these serviceable bearings or just toss and replace? Anything I should know before pulling the fork?

If your bike is this: 2008 2.3 WSD - Bike Archive - Trek Bicycle

Then the headset is listed as an Aheadset semi-cartridge. Aheadset: AHEADSET :: Bicycle Headset Manufacturers | Bicycle Headset Assembly Videos

You can eyeball those and the Park tools site for info. These are not particularly challenging to R&R. You can clean and relube
all the contact points between the headset and the frame and see how the bearings feel when removed. Cartridge bearings in
low load scenarios like headsets should last forever, however the grease can dry out over time and make the bearing noisy or
stiff. You can pop out the plastic bearing seal and clean out and repack the bearing with grease and replace the seal fairly easily.
Tracking down sqeaks can be difficult. You should also consider seatpost/seat tube interface and sometimes the saddle clamps
and saddle rails can be noisy.

To bring this one to a bit of closure. The bearings installed by Trek are manufactured under Cane Creek license to VP Components. The product number is VP-A61ACK which comprises of two MH-P03M bearings. They are basically the cane creek 41 standard with one difference, the angles are 45/45degrees as opposed to their standard of 45/36 degrees. Bottom line is finding those bearings is a nightmare and if you do, they are way more expensive than they should be. Sounds like Trek did this for a few short years and then switched to the true cane creek standard.

The solution is to put an IS41/28.6 IS41/30 new headset in. Trek sells / lists the Headset IS41 as the replacement which uses standard cane creek 41 bearings. You can also install the Cane Creek 40 series or 110 series IS41 to replace the bearings.

Wasn't that 45/45 degree bearing configuration used by Campagnolo in their integrated headsets a few years ago?

Yes, but the bearing OD is 41.8mm with the Campy standard.